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 in 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 : 



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 pari 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. 



