CRUSTACEA SEXUAL DIMORPHISM 



377 



The sexual differences are all to be ultimately explained as adaptations for 

 ensuring reproduction and for preserving the young. Adaptations facilitating the 

 copulation of the male and female are principally found in the body of the male. 

 Adaptations for securing the 

 favourable development of 

 the eggs are met with in the 

 female. 



Male Sexual Character- 

 istics. (a) The males of the 

 Crustacea are throughout 

 smaller and often also more 

 agile than the females. This 

 distinction of size k .specially 

 remarkable in parasitic and 

 attached Crustacea, where 

 the minute males (as in the 

 Cirripedia and parasitic Iso- 

 poda) are described as dwarf 

 males. On the other hand, 

 in all cases where the females 

 are so deformed and degraded 

 by parasitism as to be hardly 

 recognisable or even alto- 

 gether unrecognisable as 

 Crustaceans, the males ap- 

 pear less degenerated ; they 

 are usually still able to move 

 freely, are provided with dis- 

 tinct limbs, and show some 

 resemblance to the nearest 

 related free - living forms. 

 This slighter degeneration on 

 the part of the males may 

 be considered as the persist- 

 ence of a larval stage, the 

 parasitic Crustacea under- 

 going, as we shall see, 

 a striking metamorphosis. 

 Free-living larval forms with 

 Crustacean characteristics, 

 however, bring about the 

 infection of new hosts The FIQ 252 ._ Lernaeascus nematoxys . Aj Femal B ^ 

 degradation and crippling of more highly magnme d. a lt Anterior; 03, posterior antenna; 

 md, mandibles ; mf, maxillipedes ; 6j, b%, & 3 , ti, t%, t& thoracic 



feet ; od, oviduct ; ov, ovary ; Ted, cement gland ; ab, abdomen ; 

 d, intestine ; III, IV, V, thoracic segments ; aj, 1st abdominal 

 segment; gp, genital plate; vd, vas deferens ; t, testes (after 

 Glaus). 



the body only take place after 



the larva has attached itself 



on its definitive host. In 



the males this degradation 



does not occur, or not to 



the same extent, because they are obliged to retain their power of free locomotion in 



order to ensure the possibility of copulation and of fertilisation of the females. 



Since, however, the sole work of the male consists in the seeking out of, and the 



fertilisation of the female, we often find (as in the parasitic Cirripedia) great 



