bo 
42 BULLETIN: MUSEUM OF COMPARATIVE 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 criste acustice, and to clearly show how 
they 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 chitin, 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 
