PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 209 
into a cup-shaped depression and so labilely fastened to the chitin of 
the sac wall (Plate 10, Fig. 51) that the hair can sway freely in any 
direction, as if it were attached by a ball-and-socket joint. This cup- 
like depression is characteristic of all the otocyst hairs of Brachyura. 
The hook hairs are present in the otocyst of the Megalops larva of 
Carcinus, and are there relatively much larger ; they extend over a large 
portion of the posterior end and floor of the sac, the curved row of 25 to 
30 hairs occupying two-thirds of its length. As the otocyst is open at 
this stage, it contains numerous otoliths, and these are either im contact 
with, or attached to, the tips of these hairs. Measurements of a number of 
these larval hairs were made in the Megalops and the stage succeeding 
it, and a comparison of these with the same hairs of adults is made in 
the following table : 
: Average Length Average Diameter 
ma ot of ten Hook of ten Hook 
YBbe Hairs. Hairs, 
Adult (80 cm. long) 1.96 mm. 49 u 
Young crab 0.24 mm. AT u 
Megalops larva 0.21 mm. 46 u 
This table brings out the interesting fact that the hook hairs of a 
Megalops larva, of a young crab and of an adult are of nearly equal 
size, although the otocyst of the adult is nearly ten times as long as 
that of the Megalops, and over eight times that of the young crab. On 
measuring the thread hairs to see if the conditions there were the same, 
it was found that in the adult they were three and a half times as long 
as in the Megalops stage; the thread hairs thus more than tripled their 
length, while the hook hairs remained constant. The number of hook 
hairs is approximately the same in the Megalops otocyst and in the sac of 
the adult. Their arrested development may be explained by the fact 
that they are true otolith hairs; when the otocyst becomes permanently 
closed, otoliths can no longer enter the sac, and these hairs, as they lose 
their original function, do not grow part passu with the other hairs of 
the otocyst, but remain unchanged. They do not degenerate and 
become entirely functionless, for they are still innervated in the adult 
crab, and, though sac after sac is shed and new ones formed without an 
otolith’s finding its way into the organ, they still retain the peculiar 
form of the original otolith bristles. 
