202 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The shaft, as already noted, is nearly straight, but it is attached to 

 the floor of the sac in such a way as to make a very small angle with 

 its surface, being, in fact, nearly parallel to it. Thus in Cambarus the 

 bending has taken place at the base, not, as in Palijemonetes and 

 Crangon, in the shaft itself. In these two forms the tendency of the 

 shaft to bend must be aided, if not caused, by the weight of tlie otoliths 

 attached to the slender tips of the hairs. In the lobster and crayfisli 

 the modihed form of the shaft makes it too rigid to thus give way, and 

 the bending, if any, must take place at the thin, membranous basal 

 sphere. 



d. Formation of Hairs. (Not studied in Cambarus.) 



e. Otoliths. These are composed of large grains of sand distributed 

 mostly within the circle of hairs, and supported in part by them. As 

 the sac has a large opening, they are readily taken in through it after 

 each ecdysis. 



2. Innervation of the Otocyst. 



As the crayfish was well adapted for work with methyleu blue, a 

 large number of preparations of the sensory nerve elements were made, 

 not only of the hairs of the otocyst, but also of the other sensory 

 bristles. The nerve supplying the otocyst issues from the ventral 

 surface, instead of the anterior eud, of the brain, and at once passes 

 forward witli a slight lateral curvature to the pointed posterior end of 

 the sac, beneath which its fibres spread out to the different hairs. It 

 divides roughly into two strands, one of which passes obliquely forward 

 and mediad to supply the median set of bristles (Plate 8, Fig. 40), 

 while the other follows the course of the lateral sickle-shaped set, lying 

 on the concave side of the two rows, to which it gives off fibres along 

 its whole course. Before this division of the nerve takes place, a few 

 large fibres run out from it on the lateral side (Fig. 40) to supply the 

 short transverse row of large bristles (Plate 7, Fig. 33). 



The sensory nerve cells lie immediatel}'^ beneath the hypodermis, and 

 their peripheral fibres run in a plane parallel with the floor of the sac. 

 In the case of the transverse row of large hairs, the nerve cells are 

 situated about 450 fi posterior to the bases of the shafts, their peripheml 

 fibres being therefore nearly half a millimetre in length. This is 

 accounted for by the position of the new hair tube during tlie period of 

 its formation between moults, when it extends back from the base of 

 the functional shaft 350 fx ; the distance from base of hair to ganglion 

 cell must consequently be somewhat greater than this. 



