320 



ZOOLOGY. 



furnished each with a nerve-ending. In the gelatinous fluid 

 are solid particles grains of sand and other foreign particles 

 presumably wilfully placed there by the crayfish itself. The 

 analogy of this structure to the Vertebrate ear in a sirnplitied 

 condition is striking, that also originates as an open sac, 

 it contains sensory "hairs," and otoliths in a gelatinous 

 fluid. Hence this organ has been universally termed the 

 auditory organ of the crayfish. But when we remember 

 that the Vertebrate ear has the function of equilibration as 

 well as that of hearing, we must allow the possibility that 

 this sac in the crayfish may have the former function as 

 well as, or even without the latter. 



Many setae on the appendages, especially the antennules 

 and antennae and the mouth-appendages, are tactile ; and 

 there are certain peculiarly shaped setae on the antennules 

 which are supposed to be sensitive to odours. 



25. Reproduction. As we have already seen, the sexes 

 are distinct, and have externally-visible differences. The 

 gonads of both are similar in shape and position. It is 



hard to say if they should 

 be called median or paired, 

 each being a hollow sac, 

 paired in front and median 

 behind (figs. 162 and 163). 

 It lies between the heart 

 and the proctodaeum. The 

 ducts are paired and quite 

 continuous with the gonads, 

 from which they come off on 

 either side at the junction of 

 the median and paired por- 

 tions. The oviduct is thin- 

 walled, short and straight, 

 running down to its opening 

 on the basal joint of the 

 second walking-leg. The vas 

 deferens is long and coiled, and opens at a similar point 

 on the fourth walking-leg. 



The spermatozoa are unique among those of our types in 



Fit 



162. REPRODUCTIVE ORQAKS OF 

 MALE CRAYFISH. 



(After Huxley.) 



