242 bulletin: museum of compakative zoology. 



heavier than the liquid contents of the otocyst, in which case they would 

 be affected by gravity directly, and exposed to a greater or less pull 

 according to their different positions in the sac. My observations made 

 on dissections of fresh material of both young and adult crabs, do not 

 confirm the first of these hypotheses. The hairs rarely, if ever, float 

 upright in the fluid of the otocyst ; on the contrary they usually project 

 out horizontally, with their tips a little lower than their bases ; and 

 such conditions would favor the second supposition, that they are heavier 

 than the surrounding fluid. Unfortunately, when fresh material was at 

 hand, my attention was directed toward other problems, and no dissec- 

 tions or observations were made with the settlement of this question 

 primarily in view. It is, however, a point well worth future experimenta- 

 tion, for the function of these hairs is apparently similar to that of the 

 auditory hairs of the vertebrate cristas acusticse, and to clearly show how 

 thev are stimulated would throw light on an important problem in the 

 physiology of the vertebrate ear. 



SUMMARY. 



1. The cuticular lining of the otocyst, found in the basal segment of 

 the antennule of all decapod Crustacea, is cast with the test at each moult. 

 It is composed of thin chltin, and is suspended from the dorsal wall of 

 the antennule, which presents an aperture in Macrura, in the larval stages 

 of Brachyura, and also in adult Brachyura directly after ecdysis. 



2. In Macrura a single sensory prominence is present, either on the 

 floor or sides of the sac. In Brachyura there are three sensory regions. 

 The sensory hairs are borne upon these cushions, usually in curved rows. 



3. The otolith hairs are heavily fringed, often bent or hooked. In 

 Macrura they are attached to the wall of the sac by a thin bulb of 

 chitin ; in Brachyura the base of the hair shaft is inserted into a cup- 

 like depression ; both methods of attachment allow the hair to sway 

 freely upon its base. 



4. The free hairs of the otocyst, found in the lobster and all Bra- 

 chyura, are extremely long and attenuate ; their basal attachment is deli- 

 cate, and renders them much more sensitive than the otolith hairs. 



5. All sensory hairs are formed as double-walled tubes by numerous 

 matrix cells situated beneath the hypodermis, from which they originate. 

 After ecdysis processes from these cells extend into the shaft of the 

 newly formed hair. In preparation for the next moult these processes 



