The marine flora and fauna of the Isles of Scilly: Free-living Plathelminthes ('Turbellaria')

Hitherto only three turbellarian species have been recorded from the Isles of Scilly, Oligocladus sanguinolentus (Quatrefages 1845), Prostheceraeus vittatus (Montagu 1815) and Leptoplana tremellaris (Müller 1773). A taxonomic collection survey in May 2002 revealed 67 species of Turbellaria. Among these, 46 species are known to science and could be determined because of their sexual maturity. Six species are new to science. The new genus Scillyvortex gen. nov. is established within the Provorticidae (Dalyelliida). Two species could only be taxonomically determined to genus level, Prognathorhynchus sp. and Polycystis sp. (Polycystis naegelii group, Kalyptorhynchia). Fifteen species (Proseriata six species, Kalyptorhynchia four species, Dalyelliida two species and Acoela, Macrostomida and Typhloplanida one species) were juveniles and could not be determined to species level. Pelophila lutheri is transferred to the family Convolutidae (Acoela) based on the presence of a cirrus of duplex type. Pelophila pachymorpha is given the status of type species of the new genus Pelochildia gen. nov. Remarks are given for all the known species that display varying degrees of difference from their original description.


Introduction
The Isles of Scilly have been selected as one of only six sites through the whole of Europe for the production of an All Taxon Biodiversity Inventory (ATBI) of the marine biota (Warwick et al. 2003), by virtue of the relatively pristine environment and the comprehensive inventories of various components of the biota that are already available. The latter is largely due to the current series of papers published in this journal. The purpose of such an inventory will be to act as a baseline biodiversity standard for undisturbed marine environments in Europe, and to form the basis for calibrating techniques for the rapid assessment of biodiversity. The present paper is a further step towards this goal.
Among the meiobenthos, most attention has usually been paid to the hard-bodied taxa like nematodes, copepods etc. Generally, soft-bodied fauna like Turbellaria have been studied less, especially at the species level, due to the necessity of examining live material for taxonomic purposes. To date only the macroscopic species Prostheceraeus vittatus (Montagu 1815) Lang, 1884 (L. A. Harvey andP. C. Chapman, unpublished list, 1957), Oligocladus sanguinolentus (Quatrefages 1845) Lang, 1884 (see Smith andGault 1983) and Leptoplana tremellaris (Mü ller 1773) Oersted, 1843 (see Faubel 1983) have been recorded from the Isles of Scilly. More extensive studies performed on the neighbouring coasts of the Celtic Sea, Irish Sea and English Channel have been the subject of several taxonomic surveys (Gamble 1893(Gamble , 1900Southern 1936; Marine Biological Association of the United Kingdom 1957; Boaden 1963aBoaden , 1963b. The present paper deals with the taxonomic determination of free-living Turbellaria of the eu-and sublittoral area, mainly around St Martin's. In total, 67 species could be discerned as valid species. Of these species, however, only 46 could be determined to species level because of their sexual maturity. Six species are new to science and will be described. Fifteen species were sexually immature and could only be determined at higher taxon level (Table I).

Material and methods
For studies on free-living Plathelminthes, sediment and aufwuchs settled on Laminaria holdfasts, algae, Porifera and Cnidaria were qualitatively collected from eulittoral and sublittoral areas, mainly from St Martin's and the neighbouring area. Sampling was performed over the period from 12 to 16 May 2002. Sublittoral samples were taken by scuba diving. The freshly collected sediment and aufwuchs were transferred to boxes and plastic bags approximating ambient conditions. The extraction process of sediment samples was carried out using the seawater-ice method after Uhlig (modified by Schmidt 1968). The aufwuchs collections were sorted under a dissecting microscope. Sexually mature specimens of Plathelminthes were studied alive and in squash preparation, i.e. flattened under the increasing pressure of the coverslip as the preparation dried. Measurements of living specimens were made from videotapes of squashed individuals. All other measurements were made from the holotype. These measurements are given in parentheses in the text (m.HT/m.VS5measurement from holotype/voucher specimen). Measurements were made with the imaging analysis software AnalysisPro 3.0 (SIS Mü nster, Germany).
For detailed information on the histological methods used see Faubel et al. (1994). For preparing whole mounts, specimens were relaxed in 7% MgCl 2 and embedded in polyvinyl-lacto-phenol (PVL). Types (holotype: HT, voucher specimen: VS) are deposited in the Natural History Museum, London, as whole mounts (WM), serial sections in the sagittal plane (SSP) or CD-ROM storages.
The quantity of specimens observed is given as follows: one; two; few specimens5three to five; several specimens56-10; abundant5more than 10.

Localities
All site names given below conform to those on Admiralty Chart SC 34 (2000).
Site 1: St Martin's, Lawrence's Bay: eulittoral semi-exposed sandy beach at mid-tide level, medium to coarse sand mixed with shells, sandy flat covered dominantly with the brown algae (Fucus serratus, Fucus vesiculosus). Site 2a: St Martin's, Lawrence's Bay, eulittoral sand flat at mid-tide level, medium sand, with Arenicola marina burrows.

Remarks
Simsagittifera schultzei was described by Schmidt (1852) only on morphological characters, i.e. outline of body, colour (green pigment, reddish brown rhabdoids) and bilateral testes. Later on, an essentially more informative account was given by von Graff (1905) of the morphology and anatomy of S. schultzei. Von Graff described sagittocysts as being 45-50 mm long, and about 20 of them distributed only in the posterior part of the body. The male reproductive system consists of a short penis, ovoid or roundish, and a caudal seminal vesicle. Female organs were not observed. The description given by von Graff agrees well with the morphology and anatomy of the specimens collected. The only exception is von Graff described the presence of red eyes. In our material eyes are absent, just as stated by Schmidt (1852). In 1975, Dö rjes and Karling synonymized three serially sectioned specimens deposited by Westblad in the collection of the Swedish Museum of Natural History (SMNH) with Sagittifera sagittifera (Ivanov 1952) on the basis of the structure of the copulatory apparatus. This synonymization is incorrect because the sagittocysts of Sagittifera sagittifera are distributed characteristically both in the hind body and around the female opening and, in addition, in the mid-body bilaterally. A re-investigation of the serial sections made by Westblad proves that the three species of the SMNH identified by Dö rjes and Karling (1975) are identical with Simsagittifera schultzei. The sagittocysts are only arranged lateral and posterior of the male copulatory apparatus. The number of sagittocysts is highly variable. In the section series SMNH 49079 the number runs to about 18 sagittocysts. In the section series SMNH 48080, sagittocysts are not developed though the specimen is sexually mature. The inner anatomy agrees well with the descriptions given by Schmidt (1852) and von Graff (1905), the exception being the presence of eyes mentioned by von Graff. The section series SMNH 48081 is incomplete.

Remarks
Haplogonaria syltensis is known from habitats of sand mixed with shells in 8 m water depth (Helgoland), and of pure sand in eulittoral and sublittoral areas of the islands of Sylt and Rømø. The latter eulittoral habitats are comparable to the localities at St Martin's. Morphological differences, however, could be observed between the populations of the North Sea areas and populations of St Martin's in the development of rhabdoids. The North Sea populations have weak rhabdoids. Often, the existence of rhabdoids was not easy to ascertain when squashed under a cover glass. On the contrary, the rhabdoids of the St Martin's populations were very clear to discern and, moreover, the rhabdoids were more abundantly distributed over the body surface. All the other morphological characters looked identical, as well as the greenish-tinged central digestive tissue.

Etymology
The specific name refers to the occurrence of the species in tubes of Arenicola marina L.

Description
Length of body of sexually mature specimen up to 1.2 mm; maximum width anterior to mid-body ( Figure 1); posterior body-third tapering to a pointed end. In transmitted light Free-living Plathelminthes of the Isles of Scilly marginal parts of body yellowish grey; digestive parenchyme strongly granulated contrasting dark grey. Frontal organ present; from frontal tip of body prominent coiling glandular ducts run caudad, the glands of which lie postero-lateral of the statocyst. Statocyst 130 mm distant from frontal tip. Rhabdoids and eyes absent. Mouth opening at the transition from first to second body-fourth.
Reproductive system. Bilateral testes ventral at transition from second to third body-fourth.
Single ovary anterior to mid-body. Maturing oogonia are successively displaced caudad into bilateral rows. The seminal vesicle enters the male atrium proximally via a very short duct. At the beginning of the last body-fourth, the male atrium opens to the exterior (about 290 mm from hind end). Seminal bursa anterior to the male copulatory organ; any insemination apparatuses absent. Genital pores separate.

Discussion
Haplogonaria arenicolae was found only once and could only be observed in squash preparation. Therefore, serial sections or whole mounts could not be prepared. The morphological structures were stored on a CD-ROM on which the following description of the morphology of H. arenicolae is based.
Up to now the genus Haplogonaria contains 13 valid species and a species nomen nudum, Haplogonaria sp. 1 Yamasu and Okazaki, 1987. The present species is a valid member of the genus Haplogonaria based on the characters of a single ovary, seminal bursa without any insemination apparatuses and lack of a penis or cirrus. With respect to the structure of the seminal bursa, without insemination apparatuses, and absence of rhabdoids, H. arenicolae closely resembles H. elegans Faubel, 1976. The most conspicuous characters of H. arenicolae are the single ovary, generating germ cells which are displaced successively into bilateral rows of oogonia maturing to oocytes, separate gonopores and the much larger body (cf. Faubel 1976b). Both the species H. elegans and H. arenicolae preferentially inhabit the deeper layers of oxic sediments, particularly in the oxidized layers of tubes of Arenicola marina L.

Material examined
Site 2b: few specimens, in squash preparation.

Remarks
The rhabdoids of the St Martin's' population are not so distinct as the rhabdoids from the population of the Island of Sylt.
Family CHILDIIDAE Dö rjes, 1968 Remarks Hooge (2001) revised the family Childiidae Dö rjes, 1968, which is now characterized by the new diagnostic character, the presence of a reversed body wall musculature, i.e. with outer longitudinal muscles and inner circular muscles (Childia v. Graff, 1911 andParaphanostoma Westblad, 1942). All the other genera of the former Childiidae have been transferred to the new family Actinoposthiidae Hooge, 2001. We prefer to use the diagnosis given by Dö rjes (1968a) because we have some evidence (Faubel unpublished) that a partial or total reversal of the body wall musculature is also present in species of other families, and in particular in Haplogonaria syltensis Dö rjes, 1968 andActinoposthia biaculeata Faubel, 1974. This reversal probably depends on the degree of exposure of the habitats where the species are living.

Material examined
Site 7: one specimen, in squash preparation.

Material examined
Site 1: two specimens, in squash preparation.

Material examined
Site 1: several specimens, in squash preparation.

Remarks
Philomecynostomum lapillum is characterized by numerous hyaline crystals of polygonal shape distributed in the body laterally (Dö rjes 1968a;Faubel 1974a). In specimens of the population of the Isles of Scilly, these characteristic polygonal crystals were not developed. Obviously, it seems that these crystals are not so characteristic as documented from the North Sea population. Sometimes, however, small hyaline globular crystals could be discerned which were distributed over the whole body, in other specimens again only over the anterior body-half or only over the hind body part. The question arises as to whether these small globules are homologous with the characteristic polygonal ones of the North Sea specimens. The specimens found in Lawrence's Bay on St Martin's, however, could confidently be determined based on the very characteristic male copulatory organ in combination with a single ovary and simple seminal bursa.

Remarks
From the type locality of the Island of Sylt, specimens attain maximum length of 1.4 mm when they reach the status of senility after the reproduction period. In comparison, specimens of the population of the Isles of Scilly attained lengths of more than 2.0 mm. The colour of the body became very dirty grey, suffering loss of transparency.

Remarks
The determination of Microstomum papillosum could be very erroneous if the specimens had not reached full male maturity. In pre-mature specimens the stylet lacks the distal hook-like bend and, therefore, the species could be mistaken for Microstomum septentrionale Sabussow, 1899(cf. Faubel 1974b. Riedel, 1932 Microstomum tortipenis Steinbö ck, 1938: 7, Figure 4a-c.

Material examined
Site 15: two specimens, in squash preparation.

Remarks
The specimens collected, like the population of the Island of Sylt, did not possess rhabdites. Riedel (1932) and Westblad (1953), however, discerned rhabdites in their material collected from Disko Bay and Gullmarfjord, respectively (cf. Faubel 1974b).

Etymology
Species name derived from its sublittoral habitat.

Description
Length of body of living sexually mature specimen up to 2.54 mm, when squashed under a cover glass; maximum width in mid-body, area of oocyte ( Figure 2). Outline of body with characteristic macrostomid-like anterior end; posterior end rounded with several prominent adhesive glands (Figure 2A). Around margin few, very weak sensory hairs of varying length and stiffness present. In transmitted light body greyish translucent with yellowish brown intestine. Frontal glands present reaching anterior level of brain. Glandular ducts open at the frontal tip separately. Lateral to the pores of the glandular ducts, fields with dark granulations of unknown function. Very small eyes present, 270 mm from anterior margin of the body; distance between eyes 306 mm. Anterior margin of crescentic brain 207 mm distant from anterior end. Rhabdites with bundles of three to six rods distributed over the dorsal and ventral body surface. Digestive system with pharynx simplex pierced by extrapharyngeal glands. The intestine fills the median parts between testis and ovary and extends caudad over oocytes, ending up laterally at the level of the male system. Mouth behind brain, 390 mm distant from anterior end. Male and female pore 465 mm distant from each other.
Reproductive system (Figure 2). The male system is typically macrostomid-like. It consists of ventro-lateral testes, vasa deferentia running caudad lateral of the intestine, a seminal vesicle, a reduced prostatic vesicle and a male stylet which projects into the male gonopore. Prostatic glands surround the proximal part of the stylet, the ampulla-like distal ducts of which are encased by the proximal enlargement of the stylet ( Figure 2C  The female system could be incomplete in that only the bilateral ovary and the female pore could be discerned. The female pore is surrounded by cement glands. A seminal bursa, not clearly discernible, probably lies immediately caudal close to the oocyte.

Discussion
Within the Macrostomidae the penis stylet has been recognized among others as a main diagnostic feature. Most of the stylets of the Macrostomidae are bent or flexed. The methods of measurement of the stylets, however, have not always been consistent, and in several cases the validity of a species is often not easy to decide although the stylet is known.
Based only on the outline of the stylet, Archimacrostomum sublitorale sp. nov. belongs to the Macrostomum hystricinum group (Beklemishev 1951) having flexed stylets (see Rieger 1977: 212-213). However, that group proves not to be a phylogenetic unit. The M. hystricinum group represents an accumulation of different forms distributed in freshwater, brackish and marine water habitats. Type of the M. hystricinum group is M. hystricinum hystricinum Beklemishev, 1951 distributed in freshwater habitats of the palaearctic, near east, oriental and nearctic regions.
In 1954, Ferguson established the genus Archimacrostomum for all species having a reduced prostatic vesicle encased by the proximal enlargement of the stylet. Consequently, the species Macrostomum beaufortensis Ferguson, 1937, M. hustedi Jones, 1944, and M. pusillum Ax, 1951, M. rubrocinctum Ax, 1951, M. brasiliense Marcus, 1952and M. peteraxi Mack-Fira, 1971 had to be transferred to Archimacrostomum Ferguson, 1954. Uncertainty of membership to Archimacrostomum remains for M. hystricinum marinum Rieger, 1977 because Rieger presents no information as to whether the prostatic vesicle is free or reduced. Because of the reduced prostatic vesicle, the new species Archimacrostomum sublitorale represents a member of this genus. All archimacrostomid species live in the marine environment.
Rieger (1977: Figure 3) illustrated stylets of different marine populations from Italy (Fiascherino, area near La Spezia, and Venice) and Croatia (Dubrovnik) with outlines comparable to the stylet of Archimacrostomum sublitorale. But because information was not given on the anatomy, in particular of the male system, of the individual populations, an affiliation is impossible to any species or genus.
In comparison with the species of the genus Archimacrostomum, A. sublitorale stands out through its general body magnitude and stylet measurements.

Etymology
The specific epithet refers to the Isles of Scilly where it was found.

Description
Length of body of living sexually mature specimen up to 2.1 mm, when squashed under a cover glass (Figure 3). Outline, colour, arrangement of rhabdites and rhammites, and posterior adhesive papillae of the body correspond to the descriptions of the phena of Bradynectes sterreri given by Rieger (1971) and Faubel (1974b). Eyes, tactile cilia and adhesive glands along the lateral sides and head are absent. Pharynx, pharyngeal glands and gut disposed as in the phena described by Rieger (1971), Faubel (1974b) and Martens and Schockaert (1981).

Discussion
Rieger (1971) Martens and Schockaert (1981) discerned a fifth form, the Scheldt form. Both latter forms differ distinctly from the forms discerned by Rieger (1971) based on stylet shape and location of ovary between testis and intestine. Because of the constant location of the ovary between testis and gut, the forms of Faubel (1974b) and Martens and Schockaert (1981) represent true species: Bradynectes syltensis Faubel, 1974 and Bradynectes scheldtensis Martens and Schockert, 1981. Bradynectes scheldtensis differs essentially from B. syltensis through the absence of a sphincter around the vas deferens and the shape and dimensions of the stylet. Bradynectes scilliensis found in a rockpool at White Island is identical with B. syltensis with regard to the presence of sphincters around the vas deferens. In B. syltensis, the seminal vesicle has only a proximal spincter but the seminal vesicle of B. scilliensis is provided with a proximal and distal spincter. The male stylet ( Figure 3C) resembles that of B. syltensis, but with essential differences in shape and dimensions. Measurements of the length of the axis of the other forms were not given by the authors.

Etymology
The specific epithet refers to the Isles of Scilly where it was found.

Description
Length of body of living sexually mature specimen up to 2.9 mm, when squashed under a cover glass. Body ends rounded but rear body end somewhat pointed. Colour greyish with darkly contrasting gut and ovary. Intestine taking on yellowish brown coloration when diatoms ingested. Eyes, rhabdites, adhesive glands and tactile cilia absent around margin of body. Rhammite tracks running from anterior tip of body to the level of pharynx bilaterally. The species feeds on small nematodes, nauplii of harpacticoids, diatoms and detritus particles.
Reproductive system. Single testis and ovary on the left side of body. Testis anterior to ovary. Seminal vesicle small, joined via a short intervesicular duct with the oval prostatic vesicle (198 mm long). Male stylet apparatus ( Figure 4B,C) consisting of penis stylet and glandular stylet. Penis stylet (m.HT) 172 mm long, the twisted proximal part of which is 85 mm long and distal straight tube 87 mm long; the glandular stylet is 182 mm long (m.HT). The glandular stylet is joined with a single accessory gland; the penis stylet joins the 198 mm long prostatic vesicle. The bursal sclerotized organ ( Figure 4B,C) consists of a midpiece (42 mm long, m.HT) and a mouthpiece (56 mm long, m.HT). The midpiece looks like a clog; the mouthpiece bears distally a disc-shaped terminal thickening ( Figure 4C). Sperm tubes absent. A pyriform vesicle containing sperm opens into the proximal part of the mouthpiece.

Discussion
To date the genus Paromalostomum contains nine valid species. With the exception of P. subflavum Sopott-Ehlers and Schmidt, 1974, all of them have been recorded from the North Atlantic and its marginal seas. Based on the sclerotized elements of the male organs (penis stylet, glandular tube) and female organs (mouth piece, midpiece), P. scilliensis proves to be a member of the genus Paromalostomum Meixner in Ax, 1951. According to the arrangement of details of the sclerotized elements, P. scilliensis displays closer relationships to P. dubium (de Beauchamp 1927). Main differences from P. dubium are the absence of sperm tubes, much longer mouthpiece and diverging construction of the clog-shaped midpiece. With respect to the penial stylets, differences lie mainly in the greater length of the stylets of P. scilliensis. The male stylet looks more or less identical with that of P. dubium but the glandular tube is S-shaped ( Figure 4B). In contrast to P. dubium, P. scilliensis lacks rhabdites, tactile cilia at anterior and posterior body end, and the characteristic adhesive glands at the rear body end.

Material examined
Site 3: one specimen in squash preparation.

Remarks
The most comprehensive description of Prostheceraeus vittatus was made by Lang (1884). Later on, more valuable data were given by Bock (1913) and Prudhoe (1985) on the morphology, anatomy and the geographical distribution. The discovery of P. vittatus at site 6 on the Isles of Scilly offers the opportunity to study the anatomical structures again. This re-investigation reveals some new aspects to the anatomy of the species. Eyes are present as follows: cerebral eye clusters arranged in two elongate rows dorsal of the brain, tentacular eyes being less numerous almost exclusively distributed at the tentacular basis and the tentacles in between, and ventral eye clusters abundantly arranged halfway between anterior tip and brain. The relatively short plicate cylindrical pharynx, orientated horizontally in the pharyngeal cavity, is bell-shaped, i.e. the pharynx becomes wider distad. The reproductive apparatus matches that of P. albocinctus Lang, 1884 as presented by Prudhoe (1985: 142, Figure 129B). It is very obvious that the reproductive system of the species of Euryleptidae look anatomically very similar. The same is known for the species of Pseudocerotidae. The male atrium of Prostheceraeus vittatus is strongly provided with Figure 5. Prostheceraeus vittatus. Diagrammatic sagittal reconstruction of the male copulatory apparatus. Scale bar: 300 mm. muscles and is subdivided into a ciliated outer and inner cavity. The inner one houses the ejaculatory duct which is distally armed with a pointed stylet. The distally tapering stylet is 135 mm long and 74 mm in diameter at its base (m.VS) (Figure 5). At a distance of 72 mm (m.VS) proximad from the base of the stylet, the prostatic duct branches off from the ejaculatory duct. The prostatic vesicle is enclosed in a 12 mm (m.VS) thick muscle wall. The glandular inner lining is smooth; the maximum thickness 76 mm (m.VS). The longitudinal axis is 530 mm (m.VS). Extravesicular glands are not present. The measurements were made from serial sections.

Description
Length of body 3.4 mm, width 1.4 mm; terminal ends broadly rounded, tapering slightly anteriorly, with smooth dorsal surface ( Figure 6). Colour yellowish white in incident light; in transmitted light, margins transparent but central body dark yellowish brown when gut diverticula full of food. Marginal tentacles less prominent forming mere blunt projections of the margin, with eye clusters at their base (8-10) and a few eyes (four) in between. Cerebral eyes in bilateral clusters above brain, each with 9-11 eyes ( Figure 6). Two characteristic pairs of eyes at the posterior margin of brain. Cylindrical pharynx immediately behind brain; intestinal trunk extending from proximal pharynx to hind end with nine pairs of intestinal lateral branches; frontal median branch of intestine extends dorsally of the pharynx to the anterior end (not drawn in Figure 6). Sucker in mid-body. Specimen not sexually mature.

Remarks
To date five valid species of the genus Stylostomum are known. Of these only Stylostomum ellipse is known from European Seas. All the other species were recorded from the Southern Ocean (Iles de Kerguelen: S. frigidium Bock, 1931) and Pacific Ocean. S. ellipse is obviously highly variable in colour depending on uptake of food. That variability led Lang (1884) to Geographical distribution. Skagerrak: Sweden (Gullmarfjord: Blåbergsholmen), Norway (Oslofjord: Drøbak); English Channel: England (Plymouth: Cawsand Bay); Mediterranean Sea: Adriatic Sea (Croatia: Rovinj) (Westblad 1956).

Remarks
Differences from the description of Westblad (1956) include the dark brown pigment streaks, eyes, pharynx and seminal vesicle. In the specimen recorded from the Isles of Scilly, the pigment streaks are not very distinct and mainly developed in the area of the brain and pharynx. Only a very weak streak is seen extending backwards. There are two eye spots at each side but very close to each other simulating a single one. The pharynx does not contrast very distinctly from the surrounding tissue. In the male system the seminal vesicle is much more developed than reported by Westblad. It takes up almost the whole part of the second body-third, being frontal of the male penis sack.

Material examined
Site 12: one specimen, in squash preparation.

Remarks
Differences to the descriptions of Westblad (1955) and Karling (1993) include the length of body (1.3 mm long), length of copulatory organ (304 mm long) and two pairs of eyes.

Material examined
Site 1: few specimens in squash preparation.

Remarks
About 16 needles in a half circle enclosing five longer needles, three lateral stronger needles obliquely arranged to the 16 needles.

Material examined
Site 3: two specimens, in squash preparation.

Remarks
Promesostoma meixneri is characterized by a male stylet having three turns proximally and a characteristic distal tip. In our specimens the distal tip of the stylet was found to be straight, with a terminal pore, with no differentiations. Because it is known that during the course of development of reproductive organs the distal tip of the stylet is developed last, it is impossible to ascertain what subspecies was found. To date there are three subspecies known: Promesostoma meixneri meixneri Ax, 1951, P. meixneri roscoffensis Ehlers, 1980 andP. meixneri ericae Faubel, 1980.

Material examined
Site 12: one specimen in squash preparation.

Remarks
During squeezing the animal was burst. The determination of the species is based on characters as follows: with two eyes, pharynx behind mid-body, germar lateral beside pharynx, bilateral testes, sclerotized stylet tubular, with a distal bend of an angle of about 90u, pointed.

Description
Juvenile specimen with body length of 1.0 mm; without eyes; proboscis with a pair of hook organs on a basal muscular plate, oval to roundish (diameter of the oval side 43 mm). Each hook organ consists of three pointed spines the bases of which are rounded and swollen (Figure 7). The median spine of the hook organ 35 mm long, the inner spine 15 mm long and the outer one 16.5 mm long. Distance between both hook organs 118 mm (distance measured between bases of median spines).

Discussion
To date the genus Prognathorhynchus comprises 13 species. Of these species two are nomina nuda, P. bacescui Mack-Fira, 1973 andP. sp. Ax and. Because of sexual immaturity information on the genital apparatus could not be presented. Based on the outline of the hook organs Prognathorhynchus sp. resembles with Prognathorhynchus dubius Meixner, 1929(according to Evdonin 1977).

32
A. Faubel and R. M. Warwick reticulated lines running from anterior end to rear end. The pigmented band broad anteriorly, becomes smaller caudad. Colour of body dirty grey. Pharynx behind proboscis in horizontal plane. A sclerotized stylet marks the transition of the distal part of the prostatic vesicle into the atrium. The stylet looks like a bottleneck with a thick-ended collar and a lateral pointed projection; teeth absent ( Figure 8C). The measurements of the stylet are: diameter of distal opening 17.8 mm, diameter of proximal opening 25.8 mm, distance between distal and proximal opening 12.5 mm and length of lateral projection 6.5 mm.

Remarks
In total, four specimens of the Polycystis naegelii group (Karling 1986) were found, two specimens of which could definitely be determined as Polycystis naegelii Kö lliker based on the following characters given by Evdonin (1977) and Karling (1986): body length up to 3.5 mm, two eyes, colour dirty greyish black, and stylet with single flagellum directed caudad. Both the other specimens differ in the following characters: length of body of sexually mature specimens 1.0 mm, two bilateral pairs of eyes, band of dark red-brown reticulated lines dorsally, and stylet with a short projection directed laterad. Evdonin (1977) stated that Polycystis naegelii has stylets with either a smooth collar, with teeth along the collar, or with one or two flagella of different size at the collar. The authors are convinced that these different forms of the stylets represent different species, as do the present species. But the authors must desist from the establishment of a new species within the Polycystis naegelii group because the sagittally sectioned series is damaged so that the reconstruction of the genital system is impossible.

Material examined
Site 7: one specimen in squash preparation.

Diagnosis
Provorticidae with bilateral testes and germovitellaria; vitelline part of germovitellaria anterior to germarium part; common genital opening in the caudal body-half. Male organ with paired seminal vesicles, the common duct of which with distal sclerotized walls. Prostatic vesicle provided with sclerotized stylet, opens independently from the ejaculatory duct into the common genital atrium. Free-living, marine. Type of the genus: Scillyvortex phytophilus sp. nov.

Etymology
The generic name is composed of the former dalyelliid genus Vortex and the place found, i.e. Isles of Scilly.

Etymology
The specific epithet refers to the phytal habitat in which specimens were living (from Greek, phyton5plant and philos5specially favoured).

Description
Length of body squashed 1.7 mm, fore-end slightly tapering to a pointed tip, rear end rounded. Colour dark dirty grey with transparent areas marking the locations of the pharynx and genital organs. With a pair of crescentic eyes; lateral eyes very close to each other. Eyes 204 mm distant from anterior margin. Epidermis with numerous small rhabdites. Pharynx doliiformis small, 290 mm long, mouth of pharynx 100 mm caudal to eyes, distal pharynx bordered with tiny papillae. Hind body end provided with four very large caudal glands with inner thread-like structure up to 120 mm long.
Reproductive system. Common gonopore in rear body leading into a voluminous genital atrium. Gonopore and distal atrium surrounded by cement glands. Testes paired, behind pharynx. Vasa deferentia join testes with paired seminal vesicles. The male copulatory apparatus consists of paired seminal vesicles and a prostatic vesicle bearing a coiled stylet. The prostatic duct and the ejaculatory duct openings obviously independent from each other but very close to each other in the genital atrium. The paired seminal vesicles combine to a common ejaculatory duct; the distal part of which increases gradually in sclerotization of the walls forming distally a collar-like border. The spherical prostatic vesicle, 140 mm in diameter and provided with a thick muscular wall, opens distally into a proximally coiled stylet making one and a third turns. The stylet is tapering distad starting at the end of the first proximal third of the twisted part. The distal part of the stylet is straight, about 100 mm long and provided with a short distal hook-like bend at an angle of about 90u. The stylet resembles the penis stylet of Pogaina annulata Ax, 1970. The female gonads are paired germovitellaria. The vitellogenous part is located dorsolaterally and extends from somewhat anterior of the testes to the level caudal of the seminal receptacle. The distal ovarial parts lie immediately in front of the seminal vesicle. Female ducts, i.e. oviduct and uterus, could not be discerned and traced to the common genital atrium.

Discussion
According to Luther (1962), the characters of paired gonads and common genital atrium in the caudal body-half refer Scillivortex phytophilus to the family Provorticidae. The Provorticidae are additionally characterized by a more or less combined seminal and prostatic vesicle. The family is subdivided into the subfamilies Provorticinae Meixner, 1926 (paired vitellaria, seminal and prostatic vesicle side by side), Kirgisellinae Luther, 1962 (paired vitellaria, seminal vesicle proximal of prostatic vesicle) and Haplovejdovskyinae Luther, 1962 (single testis and germovitellar, paired vasa deferentia). Based on the characters given, Scillivortex phytophilus differs from the known species of the subfamilies of the Provorticidae in the character combination of paired germovitellaria and separate seminal and prostatic vesicle with excreting ducts which separately open into the common genital atrium. A special feature is that both ducts are provided with sclerotized elements and enter independently from each other into the genital atrium. This character combination is unique within the Provorticidae and the other families of the order. Consequently, we have to establish a new family or at least a new subfamily within the Provorticidae. However, we must desist from this because of the absence of serial sections for the final resolution of the exact relations of the male ducts and their entrance into the genital atrium.

Remarks
The species was found in fully marine habitats. To date Balgetia semicirculifera has only been found in oligohaline habitats of the Baltic Sea (Luther 1962) and in saltmarshes of the Island of Sylt . The stylet of the Scilly form looks rather different: the proximal and median part is straight and only the distal part is curved ( Figure 9C) (cf. Luther 1962: 33, Figure 12D, E).

Description
Length of body of squashed specimen 1.2 mm; frontal end broadly rounded, tapering slightly anteriad; hind end variably tapering, distally tail-like. In transmitted light body yellowish transparent; intestinal tract contrasts dark grey with transparent areas marking the locations of the genital organs. Eyes absent. Pharynx 256 mm long, 102 mm distant from anterior end.
Reproductive system. Testis left in posterior part of body. The longish muscular copulatory bulb consists of a voluminous seminal vesicle and a small prostatic section, the glands of which are located extravesicular. The seminal-prostatic bulb is distally encased in the proximal penis stylet, being 38.6 mm long. The germovitellar is located on the right side in Figure 10. Dalyelliida sp., dorsal view in squash preparation. Scale bar: 250 mm.
the posterior body. The vitellogenous part extends frontad to the level of the mid-body. The distal ovarial part is connected with the common genital atrium. Common genital pore present.

Discussion
The monotypic subfamily Haplovejdovskyinae Luther, 1962 was established for Haplovejdovskya subterranea Ax, 1954, based on the characters of a single germovitellar and testis located in the posterior part of the body. In this respect Haplovejdovskya scilliensis agrees well with the diagnosis given. Essential differences from H. subterranea exist in the shape of the stylet. The stylet of H. subterranea is longer, 110 mm, twisted and distally hooked as against the stylet of H. scilliensis being 38.6 mm long, straight and distally pointed. Furthermore, H. scilliensis and H. subteranea live in different habitats. H. subterranea is a typical brackish water species (Ax 1956c) and H. scilliensis was found in a euhaline habitat.

Morphology
Length of body in squash preparation up to 1.1 mm; outline of body with characteristic head, 0.26 mm long, broadly rounded, separated from mid-body by a slight incision forming a circular groove; at each side, groove marked with bundles of 76 mm long cilia. Posterior body end tapering to a blunt point provided with a bundle of up to 130 mm long cilia. From time to time mid and posterior body exhibits very slight incisions ( Figure 10) simulating pseudo-segmentations. Around head and margin sensory cilia, 93 mm long, present in more or less close serial sequence. Epidermis with 39 mm long cilia, rhabdites absent. Colour yellow-red-brown. Head with two crescentic eyes, 64 mm long and 125 mm distant from each other. Pharynx doliiformis 132 mm long, opens immediately behind ventral groove. Intestine with ingested diatoms.
Reproductive system. Specimens were sexually immature.