PAM BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
nerve (n. at.1) turns straight forward, while that of the otocyst divides 
into three branches (Fig. 55, n. ot., n. ot.!, n. ot.!). The most median 
and largest of these runs forward to supply the thread hairs; the 
middle branch goes directly to the posterior sensory cushion, which 
bears the hook hairs; while the third and lateral offshoot takes a 
nearly straight course along the posterior wall of the sac and supplies 
the tactile hairs of the antennule, and possibly the group hairs of the 
otocyst. The ganglion cells of the hook hairs are some distance pos- 
terior to the hairs and arranged in an irregular scattering group 
(Plate 10, Fig. 50, el. gn.). Those of the thread hairs are lateral and 
posterior with reference to their hairs, lying immediately beneath the 
hypodermal cells of the sensory cushion, and forming an irregular single 
row, which is nearly parallel to the row of thread hairs (Plate 10, 
Fig. 53, el. gn.). 
a. Number of Nerve Elements to a Single Bristle. The nerve ele- 
ments of the thread hairs were brought out clearly and completely by 
methylen blue and by Vom Rath’s platinic-chloride method. The 
conditions found in a number of preparations are shown in Figure 53, 
where there is but a single element for each hair. This particular 
preparation was obtained with methylen blue, but the results were 
verified by Vom Rath’s method. Counted in serial sections, the num- 
ber of hairs and ganglion cells were approximately equal. 
By the same method of counting, the elements of the hook hairs 
gave like results. In one case there were thirty hairs and thirty- 
one cells. No ganglion cells could be made out near the group hairs, 
nor any fibres supplying them. Certain clusters of cells are found 
directly beneath their bases, but their large peripheral processes, irregu- 
lar outlines, and lack of central fibres marked these as matrix rather 
than nerve cells. 
Here in Carcinus, then, as in the macruran forms described, there 7s 
but one nerve element to each otocyst hair. 
The distal segment of the antennule was by chance sectioned in 
making preparations of the otocyst, and when stained with iron hema- 
toxylin, the innervation of the olfactory hairs found in that region was 
sharply brought out (Plate 10, Fig. 52). As in the examples of this 
type of hair already described, « large spindle-shaped group of about 100 
ganglion cells sends a strand of nerve fibres to the base of each shaft. 
These cells are relatively small and situated 0.5 mm. posterior to the 
hairs they supply. In Figure 52 a single nerve element is shown 
diagrammatically in black. 
