382 THE CRAYFISH CHAP. 



are minute sand-grains, which take the place of the 

 lithites or otoliths (pp. 188 and 315) found in most organs 

 of this kind ; but which, instead of being formed by the 

 animal itself, are taken in after each ecdysis when the 

 lining of the sac is shed. Many of the setae on the body 

 generally have a definite nerve-supply, and are probably 

 tactile organs. 



The crayfish is dioecious (p. 320), and presents a very 

 obvious sexual dimorphism or structural difference 

 between male and female, apart from the actual organs 

 of reproduction. 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. 364) ; and the reproductive aper- 

 ture is situated in the male on the proximal podomere 

 of the fifth leg, in the female on that of the third. 



The spermary (Fig. 93, t) 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 

 defer ens (vd), which opens on the proximal segment of 

 the last leg (vdo). The sperms differ greatly from those 

 already described in other animals : they are curious, 

 rounded, non-motile bodies produced into a number of 

 stiff processes : by a secretion of the spermiduct they 

 become aggregated into vermicelli-like spermatophores. 



The ovary is also a. three lobed body, and is similarly 

 situated to the spermary : from either side proceeds a 

 thin-walled oviduct, which passes downwards, without 

 convolutions, to open on the proximal segment of the 

 third or antepenultimate leg. The eggs are of consider- 

 able size and contain a large amount of yolk. 



Both ovary and spermary are hollow organs, dis- 

 charging their products internally. Their cavities, lined 

 by germinal epithelium, represent part of the true 



