208 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



hairs are never iu 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. A; Plate 9, Fig. 42, set!^. There is only a slight prominence, the 

 surface bearing the hairs being nearly fiat. 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. Tlie 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 

 cristce acusticce. 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 tliau 

 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) Tke hook hairs are found on the posterior vertical cushion (Fig. 

 A and Plate 10, Figs. 50, 55, set. ia.) arranged in a very irregular 

 curved row. They vary from 25 to 31 in number, and are relatively 

 very small, averaging 49 ^ in length and 4 /i 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 



