Type Material: Holotype. AMNH PLATY 1658, one set of 1.5­µm­thick serial sagittal sections of epoxy­embedded specimens stained with toluidine blue. Paratypes. AMNH PLATY 1659, one set of 1.5­µm­thick serial sagittal sections of epoxy­embedded specimens stained with toluidine blue, and AMNH PLATY 1660, epoxy­embedded whole mount.

Type Locality. San Felipe, Baja California, medium grained sand from a small pool of standing sea water at the high intertidal from the south side of San Felipe Bay (30°58’35.1”N, 114°48’22.6”W).

Other Material Examined. Living specimens in squeeze preparations; three sets of 1.5­µm­thick serial sections of epoxy­embedded specimens; several whole mounts for fluorescence imaging of musculature.

Etymology. Species name is a derivation of the Spanish amarillo, yellow, referring to the species body color.

Description. Mature, living specimens up to 650 µm long and 150 µm wide (Figs. 1A, 2). Preserved specimens contracted to approximately 360 µm long and 140 µm wide (Figs. 1B, 3). Body cylindrical. Anterior and posterior ends rounded. Body color yellow by transmitted light.

Epidermis completely ciliated. Many large rhabdoid glands present; mostly concentrated at anterior end (Figs. 1, 2).

Musculature with circular muscles that encircle the body along entire length of animal; straight longitudinal muscles present between frontal organ and anterior edge of mouth; longitudinal­cross­over muscles (fibers with a longitudinal orientation anteriorly, but bend medially to cross diagonally) present in both dorsal and ventral body wall; anterior end with ventral diagonal muscles positioned between outer circular and inner longitudinal muscles (Fig. 3A).

Frontal organ well developed; cell bodies of frontal glands positioned ~ 100 µm behind frontal pore (Figs. 1B, 2B).

Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from posterior end of frontal glands to level of seminal vesicle.

Ovary presumed to be unpaired, ventral; no more than two eggs —positioned medially—discernible in live and sectioned material (Figs. 1, 2A, C).

Testes paired, dorsal, compact; separate from ovary. Testes extend anteriorly to level of mouth and posteriorly to male copulatory organ.

Common gonopore on ventral surface opens anteriorly to sperm­filled seminal vesicle composed of thin layer of muscle and surrounded on ventral and posterior sides by several conspicuous nuclei (Figs. 1B, 2B, 3B).

Gonopore opens posteriorly to ill­defined, weak­walled vagina that runs anteriorly over dorsal side of seminal vesicle to sperm­filled seminal bursa; bursa with well­developed bursal wall (Figs. 1B, 2B).

Remarks. There are presently 14 described species of Haplogonaria that are united in having an unpaired ovary, paired testes, and a seminal bursa that lacks a bursal nozzle. Some species, such as H. arenaria (Ax, 1959), H. elegans Faubel, 1976, and H. stradbrokensis Hooge, 2003 have a non­muscular, or weakly muscular seminal vesicle, while other species, such as H. phyllospadicis Hooge & Tyler, 2003, and H. simplex Dörjes, 1968, have a strongly muscular seminal vesicle. H. amarilla is more like the former, with only a very thin layer of muscle contributing to the wall of the seminal vesicle (Fig. 3B). It is unusual for members of the Haploposthiidae to have a vagina positioned as it is in this species; that is, opening posterior to, and passing dorsally over the seminal vesicle. Within the Haplogonaria, this configuration of the vagina is shared only with H. sinubursalia Dörjes, 1968. In other regards, the vagina and bursa of H. amarilla are dissimilar to that of H. sinubursalia, which has a vagina filled with granular secretions (vesicula granulorum), and a bursa bearing a cap of cells on its proximal wall; both of these features are lacking in H. amarilla.