382 THE CRAYFISH CHAP. 



which fills the sac 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 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. 364) ; 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 spermary (Fig. 93, /) 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 differ greatly from those already considered in 

 other animals : they are curious, rounded, non-motile bodies 

 produced into a number of stiff processes : by a secretion 

 of the vas deferens they become aggregated into vermicelli- 

 like spermatophores. 



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

 ated 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 antipenulti- 

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

 large amount of yolk. 



Both ovary and spermary are hollow organs, discharging 



