224 THE LIFE OF CRUSTACEA 



have suctorial mouth parts, and lead a semi-parasitic 

 existence ; but the only completely parasitic forms 

 are the Whale-lice, forming the family Cyamidse (see 

 Fig. 23, p. 55) in the suborder Caprellidea. Although 

 differing greatly in the broad, flattened shape of the 

 body from the slender, thread-Hke Caprellidse, they 

 closely resemble them in structure, particularly in 

 having the abdomen reduced to a mere knob. The 

 fourth and fifth pairs of thoracic limbs have dis- 

 appeared, although the gills corresponding to them 

 are very large ; and the last three pairs of legs have 

 long curved claws which enable the Whale-louse to 

 cling firmly to the skin of its host. The mouth parts 

 are adapted for biting, not for sucking blood, and 

 the animals seem to live by gnawing the skin of the 

 Whales. In one respect the Whale-lice are unique 

 among Crustacean parasites : they have not the 

 power of swimming at any period of their life-history. 

 The young settle down near their parents, and masses 

 of many hundred individuals of all sizes are found 

 clinging close together on the skin of the host. 



No group of Crustacea exhibits more numerous or 

 more varied examples of parasitism than the Co- 

 pepoda. Every grade of transition between a free 

 predatory habit of life and the most complete 

 dependence upon a host may be traced in various 

 families of the subclass. Only a few examples can 

 be mentioned here. 



The commonest " fish-lice " are the numerous 



