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 Palemonetes 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 the otoliths 
attached to the slender tips of the hairs. In the lobster and crayfish 
the modified 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 methylen 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 end, of the brain, and at once passes 
forward with 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 immediately beneath the hypodermis, and 
their peripheral fibres run in a plane parallel with the floor of the sae. 
In the case of the transverse rowof large hairs, the nerve cells are 
situated about 450 w posterior to the bases of the shafts, their peripheral 
fibres being therefore nearly half a millimetre in length. This is 
accounted for by the position of the new hair tube during the period of 
its formation between moults, when it extends back from the base of 
the functional shaft 350 1; the distance from base of hair to ganglion 
cell must consequently be somewhat greater than this. 
