CHAPTER VIII 

 PARASITIC CRUSTACEA 



Some of the Crustacea mentioned in the last chapter approached 

 a parasitic way of life. The borderline between the various associa- 

 tions described and true parasitism is overstepped when one of the 

 associates begins to use the other as its food-source. True parasites 

 are found mainly in three groups, the Copepoda, Cirripedia and 

 the Isopoda. In each of these groups we find species with modifica- 

 tions of such a profound nature that without knowing the life 

 history of the animal it would be quite impossible to identify it as 

 a crustacean. Less profoundly modified parasites are found among 

 the amphipods and the Branchiura. It will be convenient to deal 

 with each of these groups separately. 



PARASITIC COPEPODS 



In the copepods one can find all stages from almost unmodified 

 external parasites to internal parasites so modified that they can 

 only be identified as copepods by their egg sacs and nauplius larvae. 

 We can begin our account by tracing such a series; this must 

 not be interpreted as a strict evolutionary sequence, because the 

 examples have been chosen from widely divergent families. 



Various members of the family Ascomyzontidae are found on 

 echinoderms. These copepods have suctorial mouthparts and are 

 assumed to feed on fluids which they extract from their hosts. 

 Apart from this modification they do not differ very much in 

 appearance from typical cyclopoids (fig. 46A). 



Many species of the Caligoidea are parasitic on fish. The body 

 differs in shape from that of a normal copepod, but there is no 

 great difficulty in recognising the systematic position of a caligoid 

 even when adult (fig. 46B). The abdomen is greatly reduced and 

 various appendages on the head and thorax are modified for cling- 

 ing to the skin or gills of the host. One member of the group. 

 Lepeophtheirus pectoralis, attaches itself to the skin of the flounder, 

 Platichthys flesus, and various other flatfish. It receives the name 



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