PARASITA. 237 



final shape. Besides the formation of a more richly segmented 

 body, in the families of the Cydopidae and Corycaeidae, the (inner) 

 accessory branch of the second antenna is lost as early as the first 

 Cyclops stage, and the mandibular palp degenerates. 



The metamorphosis of the Calanidae (Cetochilus), which has here been 

 considered as the type, is distinguished by the regular development of the 

 limbs from before backwards, but in the Harpadidae (Fig. 112) and Cydopidae 

 an exception to this regular order is found in the second maxilla (?/), which 

 in the later Nauplius stages has, indeed, begun to form, but is still in an 

 exceedingly rudimentary condition, so that the pair of limbs which follow it 

 precede it in development. We have here a parallel to the condition of the 

 maxilla in the Phyllopoda. 



B. Parasita. 



The free-living Copepoda are connected with the more specialised 

 parasitic genera by many transitional forms which mark the various 

 degrees of parasitism, and we consequently also find various stages in 

 the transformation and degeneration of the segmentation of the body. 

 It may be laid down as a general rule that the female, on account of 

 the part played by her in reproduction, shows a greater tendency 

 to adopt the parasitic mode of life, and consequently to undergo 

 degeneration of the locomotory organs, obliteration of the boundaries 

 between the segments and deformation of the body. Thus, even in 

 the little modified Sapphirina, we find that the females become 

 parasitic in the respiratory cavity of the Tunicata, or in the umbrellar 

 cavity of Diphyes, while the males are always found swimming freely 

 about. An extreme example is found in Lernaea (Fig. 114 A and B), 

 the metamorphosis of which ends in a Cyclops-like free-swimming 

 stage, during which copulation takes place ; the female afterwards 

 becomes attached to a fish (often one of the Gadidae), and undergoes 

 great deformation of the body (Fig. 114 C and D). Heteromorphism 

 of the sexes develops in this group, in so far as the male is only 

 slightly removed in the segmentation of its body from the later 

 Cyclops stages, while the female is greatly transformed in accordance 

 with her parasitic habit. The same is the case in the Philichthyidae 

 and the Chondracanthidae. In some forms, on the contrary, the 

 male also departs from the Cyclops-like shape of the later larval 

 stages by a secondary transformation. Whereas, in the Caligidae 

 and Dichelestiidae, the two sexes are not strikingly different in 

 shape of body or size, in the Lernaeopoda (Fig. 115 D and E), 

 the heteromorphous development of the two sexes takes place in 

 a way different from that shown in Lernaea. The males here 



