Thalassema

Key to the species of Thalassema from the Atlantic Ocean

1 Gonoducts two pairs................................................................................................................................................... 2

- Gonoducts one pair..................................................................................................................................................... 3

2 Gonostome fan-shaped; papillae arranged in transverse series in middle region of trunk.................. T. philostracum

- Gonostomal lips semicircular and frilled, papillae largest and most densely packed in posterior region of trunk, not arranged in transverse series................................................................................................................ T. thalassemum

3 Anal vesicles lacking; microscopic papillae located only in middle region of trunk............................ T. antarcticum

- Anal vesicles present.................................................................................................................................................. 4

4 Gonostomal lips folded or crumpled; rectal caecum absent........................................................................ T. elapsum

- Gonostomal lips not folded or crumpled.................................................................................................................... 5

5 Interbasal muscle absent; gonostomes large; papillae more closely packed at anterior and posterior ends of trunk.................................................................................................................................................................. T. arcassonense

- Interbasal muscle present............................................................................................................................................ 6

6 Integument thin and transparent; papillae small white specks scattered over trunk, more closely arranged at posterior end; rectal caecum absent......................................................................................................................... T. diaphanes

- Integument opaque; papillae prominent, unequal in size, the larger ones scale-like and bent forwards; rectal caecum present........................................................................................................................................................ T. hartmani