PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 199 



Leaving the anterior end of the brain with a bend away from the 

 median plane, it gives off in front of the globulus a small lateral branch 

 {rm. L), which supplies the tactile bristles of the antennule. The main 

 nerve, after passing between the globulus and the posterior end of the 

 sac, runs forward only a short distance to the sensory prominence on the 

 lateral side of which its ganglion lies. The peripheral fibres can be 

 traced forward and slightly mediad from the ganglion to the bases of the 

 otocyst hairs. The whole course of the nerve is approximately in a 

 frontal plane, though its peripheral ending is slightly more ventral than 

 its point of departure from the central organ. In Figure 28 (Plate 6) the 

 transverse section of the antennular nerve (n. at.l) is seen to be median 

 to the sac, while the ganglion cells of the otocyst nerve {cl. gn.) are 

 lateral to it. 



a. Number of Nerve Elements to a Single Bristle. There is in Crangon 

 but one ganglion cell and fibre to each otocyst hair. The cells and fibres 

 were counted as in Palaemonetes, and the numbers thus obtained were 

 found to agree approximately with the number of the hairs. 



Methylen-blue preparations of the olfactory nerve elements were 

 obtained, and the conditions there brought out agreed essentially with 

 those foimd in the same type of hair in Palaemonetes, large groups 

 of nerve cells being present beneath each olfactory bristle. 



h. Peripheral Terminations. Nerve fibres to otocyst hairs were 

 never traced beyond the enlarged base of the bristle, where they end free 

 without branching. A typical nerve element of the otocyst is given 

 diagrammatically in Figure 29 ; it shows the peripheral ending of the 

 fibre at the base of set. ot. 



In the olfactory hairs, on the other hand, the nerve fibres in most 

 cases could be traced up into the shaft of the hair, though never through 

 its whole length. Thus in Crangon, as in Palaemonetes, there is a 

 distinct difference in the innervation of the two types of bristles, both as 

 to the number of elements, and in the manner in which the fibres end. 



c. Central Terminations. Centrally the otocyst nerve ends in a posi- 

 tion (Fig. 29) corresponding to that of the central terminations in Pal- 

 semouetes, but the fine fibrillar branching, which was brought out 

 distinctly by methylen blue in that form, could not be impregnated in 

 Crangon. 



d. Histology of the Nerve Elements. So far as worked out, this was 

 similar to that already described in Palaemonetes. A myelin sheath is 

 present in Crangon as well as Palaemonetes, though it was not observed 

 in any other decapods. 



