PKENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 193 



3, Development of the Otocyst (in Homarus americanus Milne- Edwards). 



In order that the development of the otocyst in the lobster may be 

 more readily understood, it may be best to compare briefly its adult 

 condition with that of Palasmonetes. 



It was dissected and described by Farre ('43), and again by Hensen 

 ('63). The sac is drawn out posteriorly into a dorso-ventrally flattened 

 projection, the " cochlea " of Hensen. The external aperture is extremely 

 small, guarded by bristles, and located at the median, dorsal, and ante- 

 rior end of the sac, the dorsal wall of which, like the dorsal wall of 

 the antennule, is very thin, forming the so-called tympanic membrane. 

 On the floor, which is nearly horizontal, there is a serai-circular ridge 

 (Plate 5, Figs. 24, 26), which forms the sensory cushion. From this 

 arise the otolith hairs, which have straight shafts, and number from 

 500 to 600. The four rows of these are so arranged as to form a semi- 

 circle, the open side of which (at the right in Plate 5, Fig. 26), is ante- 

 rior instead of posterior as in Palaemonetes. At the anterior end of the 

 curve there is an irregular group of smaller hairs, with bent shafts. On 

 the median wall of the sac, near its posterior end, there is an irregular 

 double row of long thread-like hairs, with shafts heavily fringed (Fig. 

 26, set. m.). The otoliths are numerous, and rest on the area suiTounded 

 by the rows of sensory hairs, and also on the hairs themselves ; the 

 thread-like hairs are free, and float out into the lumen of the sac. 



Not much has been written on the development of the otocyst in 

 decapods. Reichenbach ('86), in his work on the embryology of the 

 crayfish, figures the invagination of the " auditory sac " at an early 

 stage in the egg. The crayfish, however, as it develops into the adult 

 form without passing through the larval stages characteristic of most 

 other decapods, is not a typical example. Herrick ('95, p. 194) alludes 

 to the appearance of the otocyst cavity in the third larval stage of 

 Homarus, and he shows its position at this stage in connection with the 

 development of the first antenna. In the fourth stage it is a shallow 

 depression containing a few otoliths and in the fifth larva its aperture 

 begins to close. 



I shall describe its condition in the first four larval stages. 



a. First Larval Stage. 



(Schizopod larva, without abdominal appendages.) 

 Sections of lobster eggs in different stages up to time of hatching 

 showed no evidence of the otocyst in the antennule, and it became 



