446 PHYSIOLOGY OF THE INVERTEBRATA. 



cells, which give rise to ovisacs. (See Pig. "jQ, g). An ovisac 

 consists of a mass of cells; a central cell, however, grows 

 until it forms an ovum (containing a germinal vesicle and 

 spot, vitellus, and vitelline membrane). The ovisac ultimately 

 hursts, liberating the ovum, which falls into the cavity of the 

 ovary. It then passes down the oviduct to the exterior, where 

 it becomes attached by a viscous matter (covering the vitelline 

 membrane) to one of the swimmerets. The viscous matter 

 hardens, and consequently encloses the egg in a tough case, 

 which is suspended from the swimmeret by a peduncle, which 

 is in reality a prolongation of the substance of the case. The 

 ova are fecundated while in the female. The fecundated 

 ovum undergoes partial segmentation of the vitellus; and 

 after the formation of a short, round, primitive streak, the 

 limbs develop, and the embryo passes through the JVaupliics 

 stage. After this the embryo develops further, and is hatched 

 in the general form of the adult, the Zocea or Copepod stage 

 being rapidly assumed and discarded during the embryonic 

 existence. After moulting, the abdominal feet are developed, 

 and the young Astacus becomes altogether similar to the 

 adult form. 



In the Macroura and Paguridce, the male genital apertures 

 are ' ' surrounded by a soft sphincter, without any trace of a 

 penis, but out of which the ductus ejaculatorius is perhaps 

 protruded during copulation. But with the Bracliyura and 

 short-tailed Anomoura, on the contrary, there are two longer 

 or shorter tubular penes, always covered by the tail, which 

 is pressed against the abdomen. In many Decapoda, the two 

 feet of the first caudal segment are transformed into pedi- 

 cellated processes (secondary penes), the extremity of which 

 is sometimes grooved. In some short-tailed Anomoura, the 

 feet of the second post-abdominal pair take part also in the 

 act of copulation, and, for this purpose, are prolonged into 

 stalk-line organs." * 



* The forms assumed by the spermatozoa of Pagurus, Pisa, and Qrapsus 

 ate illustrated in Fig. 76, p. 412. 



