182 BULLETIX: MUSEUM OF COMrAKATIVE ZOOLOGY. 



hypodermal cells, groups of cells with elongated nuclei ; these send 

 their processes into the bases of the bristles (Plate 2, Figs. 6,7). They 

 are the matrix cells, which nourish the 4iair and, as we shall see later, 

 have to do with its formation. The central region beneath the cushion 

 is occupied posteriorly by the ganglion cells of the otocyst nerve (Plate 

 2, Fig. 6, d. gn.), and anteriorly by their peripheral fibres. 



c. Structure of hairs. The hairs of the otocyst are peculiarly modi- 

 fied. Instead of being straight, as in tactile hairs, the shaft is here bent 

 out of its course about 120°, so that its distal portion makes a sharp 

 angle with the proximal end (Plate 2, Fig. 8). The shaft is very long 

 in comparison with its diameter, being from 160// to 200 ^u in length, 

 while only 3 to 6 /x in diameter at the base. The part of it above the 

 bend becomes extremely attenuate, and is heavily fringed with long deli- 

 cate projections (pinnules), which give it the appearance of a plume. 

 These fine feathery tips, which always project toward the concave side 

 of the horseshoe formed by their bases, are crisscrossed and tangled to- 

 gether in such a way as to form a wickerlike mesh, on which the majority 

 of the otoliths rest (Plate 1, Fig. 3). The hairs are not attached firmly 

 or immovably to the wall of the sensory cushion, but an exceedingly 

 thin-walled chitinous bulb intervenes between the shaft and the wall of 

 the sac. This, the spherical mevihrane of Hensen, is from 6 to 1 2 /* in 

 diameter, and allows the shaft, itself comparatively rigid, to sway freely 

 on its base, as if articulated there (Plate 2, Fig. 8, vib. sph.). 



d. The formation of hairs has already been described by Hensen 

 ('63, p. 374) in some detail. The conditions just before ecdysis were 

 figured, but the earlier stages -were not given ; so a few supplementary 

 facts may be added here. Braun ('75) verified Hensen's account of 

 Haariuechsel in the bristles of Astacus, and himself discovered some 

 new details. 



As before stated, each sensory hair is produced by a number of 

 matrix cells, which send their processes into the shaft. In newly formed 

 hairs, these protoplasmic processes extend to the very tip of the hair 

 cavity (Plate 2, Fig. 7). In preparation for tlie next moult they are 

 withdrawn nearly to the base of the hair, leaving the greater part of the 

 hair cavity empty (Plate 2, Fig, 9). At the same time the matrix cells 

 from which these processes are given off sink deeper into the tissue, below 

 the level of the hypoderniis, and with other chitinogenous cells originat- 

 ing in the hypodermis, arrange themselves about the nerve fibre of the 



