200 bulletin: museum of comparative zoology. 



3. Development of the Otocyst. 

 This was not studied in Crangon. 



III. Cambarus affinis (Say) Girard. 



The otocyst of the crayfish has been figured by only Farre ('43) and 

 Huxley ('80). The description of the former investigator was excellent 

 for the time at which it was made. Huxley alludes to the otocvst in 

 his work on the crayfish, and gives one figure showing the sensory 

 region dissected out. Hensen ('63) describes the hairs of the otocyst 

 in Astacus fluviatalis, but does not touch npon its other structures. 



1. Structure of the Otocyst. 



a. Sac. The otocyst of Cambarus (Plate 8, Figs. 37, 38), except for 

 its smaller size, resembles that of the lobster very closely. The aper- 

 ture, exceedingly small in the lobster, is here quite large, though, on 

 account of the dense chevaux de frise of fi'inged bristles, it seems smaller 

 than it really is. These bristles, projecting from around its margin, 

 effectually cover and conceal the opening. It occupies the middle of the 

 dorsal side of the antennule ; its anterior margin corresponds to the 

 anterior wall of the otocyst, and it extends back from this point nearly 

 one-half the length of the sac. Its width is about one-third that of the 

 otocyst (Fig. 37). 



The cyst does not by any means fill the cavity of the antennule. It 

 is rounded off in front, but sharply pointed at its posterior end, where 

 it is very shallow (Fig. 38). Its walls are of uncalcified chitin and 

 continuous with the very thick calcified shell of the antennule (Figs. 37, 

 38). Its dimensions in average-sized animals are : 



Length from 1.75 mm. to 2.25 mm. 

 Width " 1.52 " " 2.10 '' 

 Depth " 0.85 " " 1.05 " 



h. Sensory Cushion. The sensory ridge, or cushion, in the base of 

 the otocyst is not prominent, as that part of the sac floor upon which 

 the sensory hairs are borne is but slightly elevated above the rest (Fig. 

 38, set. ot.), and, contrary to the conditions found in the two forms already 

 described, the sensory surface is nearly horizontal, instead of being 

 vertical or oblique. The arrangement of the hairs is shown in Figure 

 40 (Plate 8). Three sets can be distinguished, corresponding to the 

 divisions of the otic nerve, — a median, a lateral, and a transverse or 

 posterior. The first and third are nearly straight, the second sickle- 



