598 THE BIOLOGY OF MARINE ANIMALS 



Three endoparasitic prosobranchs (Aglossa) will exemplify the final 

 stages of regression achieved in this group. Gasterosiphon deimatis (Stili- 

 feridae) lives inside a holothurian (Deima), but retains communication 

 with the exterior by a long siphon which penetrates the skin of the sea- 

 cucumber. This siphon swells into a bulb which encloses the visceral mass 

 of the animal, and at the opposite pole of the latter a long thin proboscis 

 extends to an intestinal blood vessel of the holothurian, from which the 

 parasite obtains its nourishment. The structure within the central swelling 

 is still recognizably a gastropod, although the foot is rudimentary and the 

 shell has disappeared. The oesophagus leads into a stomach cavity from 

 which ducts radiate out into a hepatopancreas. The walls of the siphonal 

 tube and the swelling represent a greatly hypertrophied pseudopallium. 

 Gasterosiphon is a hermaphrodite, and embryos develop in the pseudo- 

 pallial chamber. 



This animal can be advantageously compared with En toco lax and 

 Entoconcha (Entoconchidae), two forms living in the body cavities of holo- 

 thurians (Myriotrochus and Chiridota). Both parasites are mere vermiform 

 tubes, with a siphon opening through the host's skin and a proboscis pro- 

 jecting from a pseudopallial bag. Entocolax hangs freely in the body cavity 

 of the holothurian : its proboscis contains a blind oesophagus-intestine, 

 and the visceral mass is represented by female reproductive organs. 

 Entoconcha is similar except that the end of the proboscis lies in the ventral 

 blood vessel of its host, and the pseudopallial chamber is a narrow cylinder. 

 The larvae of these highly modified parasitic prosobranchs, it may be 

 noted, are typical veligers, which later undergo morphological trans- 

 formation and regression on finding a host (1). 



Pyramidellids are another group of ectoparasitic gastropods of special- 

 ized habits. The host is usually a tubicolous polychaete or lamellibranch 

 (e.g. Odostomia eulimoides on Pecten and Ostrea, and Chrysallida spiralis 

 on Sabellaria). The parasite reaches the body wall of the host by means 

 of a long proboscis, pierces it with a stylet and sucks out blood and 

 tissue debris with the aid of a buccal pump (16a, 30). 



Crustacea. Parasitism in crustaceans is very extensive and involves whole 

 orders in some instances. The modifications are most diverse and range, as 

 in gastropods, from incipient parasitism to marked degeneration. 



The isopods embody three parasitic groups: the Gnathiidae, Cymo- 

 thoidae and three families of the sub-order Epicaridea. The Gnathiidae 

 are a rather homogeneous group of isopods in which the juveniles are blood- 

 sucking ectoparasites of fish, and become free-living when mature. There 

 is marked sexual dimorphism in these animals, and the two sexes were 

 originally assigned to separate genera, Ancaeus for males, and Praniza for 

 females and immature animals, and the term praniza is still used in refer- 

 ring to the parasitic larvae. On hatching, a larva attaches itself to a fish by 

 its mandibles, and perforates its skin: neither the species of host nor the 

 position of attachment is fixed (p. 479). The blood of the host is ingested, 

 fills the hepatic caeca of the parasite and is accommodated by a great 



