36S THE CRAYFISH chap. 



setae on the body generally have a definite nerve-supply, 

 and are probably tactile organs. 



The crayfish is direcious (p. 312), and presents a very 

 obvious sexual dimorphism or structural difference between 

 male and female, apart from the actual organs of repro- 

 duction. The abdomen of the female is much broader 

 than that of the male : the first and second pleopods of 

 the male are modified into tubular or rather spout-like 

 organs (p. 350) ; and the reproductive aperture is situ- 

 ated in the male on the proximal podomere of the fifth 

 leg, in the female on that of the third. 



The speniiary (Fig. 86, /) lies in the thorax, just beneath 

 the floor of the pericardial sinus, and consists of paired 

 anterior lobes and an unpaired posterior lobe. From each 

 side goes off a convoluted spermiduct or vas deferens {vd), 

 which opens on the proximal segment of the last leg {vdo). 

 The sperms are curious, rounded, non-motile bodies produced 

 into a number of stiff processes : they arc aggregated into 

 vermicelli-like spermatopJiores by a secretion of the vas 

 deferens. 



The ovary is also a three-lobed body, and is similarly situ- 

 ated to the spermary : from each side proceeds a thin-walled 

 oviduct, ^vhich passes downwards, without convolutions, to 

 open on the proximal segment of the third or antepenulti- 

 mate leg. The eggs are of considerable size and contain a 

 large amount of yolk. 



Both ovary and spermary are hollow organs, discharging 

 their products internally. Their cavities, lined by germinal 

 epithelium (p. 336), represent the ccclome, and their ducts 

 are organs of the same general nature as nephridia (p. 331). 

 The ova, when laid, are fastened to the seta; on the 

 pleopods of the female by the sticky secretion of glands 

 occurring both on those appendages and on the segments 



