208 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
hairs are never in contact with the otoliths, even in the larval 
stages. 
A third region, on which sensory hairs are located, is found at the 
extreme lateral side of the sac, beneath the fused lips of its opening 
(Fig. 4; Plate 9, Fig. 42, set.’). There is only a slight prominence, the 
surface bearing the hairs being nearly flat. The hairs are arranged in 
irregular fashion, somewhat like the groups of otocyst bristles situated 
near the aperture of the sac in the crayfish and lobster. Numerous 
groups of matrix cells lie directly below these hairs, but no nervous 
structures could be distinguished in their vicinity. 
The great hammer-like prominence, which serves for the attachment 
of the antennular muscles, separates the sac roughly into an upper, 
anterior chamber and a lower, posterior one. The first of these com- 
partments is again partially separated into two by the anterior sensory 
prominence, which nearly meets the “hammer.” These three chambers, 
into each of which sensory hairs project, were likened by Hensen to the 
semi-circular canals of the vertebrate ear, and the sensory regions to the 
criste acustice. As the compartments are in free communication, are 
not at all canal-like in form, and are arranged in no definite positions 
relative to each other which might be of functional importance, there 
seems to be no more logical reason for making such a comparison than 
for comparing the hammer-like projection of the otocyst to the malleus. 
The apparent division of the otocyst into three compartments is not a 
modification for the purpose of increasing its usefulness as a sense organ, 
but evidently a condition brought about mechanically by the differen- 
tiation of the “hammer” along lines which would make it better 
adapted for the attachment of muscles. 
c. Structure of Hairs. The hairs, as already indicated in describing 
the sensory regions, are of three kinds. Hensen’s account of them is 
fairly good. He divides them into the following classes: (1) hook 
hairs (Hakenhaare), (2) thread hairs (Fadenhaare), and (3) grouped 
hairs (Gruppenhaare). 
(1) The hook hairs are found on the posterior vertical cushion (Fig. 
A and Plate 10, Figs. 50, 55, set. ta.) arranged in a very irregular 
curved row. They vary from 25 to 31 in number, and are relatively 
very small, averaging 49 u in length and 4 4 in diameter. Their shafts 
are hooked, often bent nearly double, and are sparsely fringed near the 
tip, if at all. The base is enlarged, as is usual in otocyst hairs, but 
not so markedly as in the forms already studied. Instead of being 
attached to a large spherical membrane, the base of the shaft is set 
