[20 A BIOLOGY OF CRUSTACEA 



they are enclosed in a membrane which is made by the crab. In 

 the later stages of infection the cavity enclosed by the membrane 

 communicates with the outside via a hole in the inner wall of the 

 host's gill chamber. The usual epicardian larval stages are passed 

 through, and the parasite enters the crab as a cryptoniscus. This 

 moults and loses its limbs so that it resembles a small maggot. The 

 blood cells of the host react to the presence of the parasite by 

 gathering around it and enclosing it in a membrane, but the young 

 parasite makes a hole in this membrane so that it can obtain food 

 from the host's blood. The male is very much smaller than the 



PART OF 6R00O POUCH 



ftureNNULe 



ANTENNA. 



5TH PLEURAL 

 FOLO 



ANTERIOR VENTRAL 

 OVARIAN PROCESS 



1ST PLEURAL POLOS 



POSTERIOR VENTRAL 

 OVARIAN PROCESS 



Fig. 52. Pinnotherion vermiforme (Isopoda, Epicaridea, Family 

 Entoniscidae). Young adult female from body cavity of a pea 

 crab (Pinnotheres pisum). Actual length about 13 mm. (After 

 Atkins, 1933). 



female and spends its life in her brood pouch. As in the other 

 epicarideans the male is quite recognisable as an isopod. 



All the parasitic isopods we have dealt with so far have been 

 parasitic on Crustacea. There are others which parasitise fish. The 

 Gnathiidae are the most interesting of these because they are only 

 parasitic in the young stages and are free-living as adults. The 

 adults are found in small groups, usually one male with several 

 females, in crevices in rocks, or in tunnels in the banks of estuaries. 

 Different species inhabit different localities: Gnathia oxyuraea is 

 found on rocky shores, and Paragnathia formica in estuaries; yet 

 other species, such as Bathy gnathia curvirostris, are only found in 

 the abyssal depths of the sea. 



