212 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



nerve (n. at.i) turns straight forward, while that of the otocyst divides 

 into three branches (Fig. 55, n. ot., 7i. otJ, 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 antenuule, 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, cl. 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, cL 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 Piath'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 Path'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 is 

 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 hiema- 

 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, a 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. 



