PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 201 
shaped. The “median” set consists of a single nearly straight row, 
running from the posterior angle of the sac obliquely forward and 
mediad, back of which there are two or three shorter, irregular rows 
of scattered hairs. The lateral set consists of two concentric rows, 
which have the form of a crescent or the blade of a sickle, the handle of 
which is represented roughly by the nerve trunk connecting the bristles 
with the brain. The hairs of the outer row are much larger than those 
of the inner series. At the tip of the sickle blade the area covered by 
the bristles expands, and the hairs are arranged in 4 or 5 irregular 
rows. Behind the proximal end of this sickle-shaped double row of 
bristles is a short row of very large hairs, the posterior set (Fig. 40, 
set. p.), usually nine in number, which extends transversely across the 
posterior portion of the sac immediately in front of its pointed base. 
Matrix cells are found in the region directly beneath the hairs, as in the 
other forms described (Plate 8, Fig. 37), and the nerve cells with their 
peripheral fibres lie below the chitin, either just within (lateral set), 
or slightly posterior to (median and transverse sets) the rows of hairs 
(Plate 8, Fig. 40). By looking down upon the floor of the sac one can 
make out numerous small pores (represented in Figure 40 by minute 
circles), which penetrate the chitinous wall in that portion of the floor 
which is inclosed by the sensory bristles, especially in its lateral part. 
In transverse sections some of these pores are cut through, and it then 
appears that they connect with the ducts of multicellular glands which 
are located in the tissues beneath. One of these glands with its duct 
and pore is shown in Figure 39. It is apparently similar to the tegu- 
mental glands found in different parts of the lobster and figured by Her- 
rick (95, Cut 5, p. 77). In Cambarus these glands evidently supply the 
secretion which attaches the otoliths to the pinnules of the otocyst hairs. 
ce. Structure of Hairs. This has been described in some detail by 
Hensen (’63), to whose descriptions I have not much to add. The 
hairs are very similar in structure to those of the lobster. Their 
number varies greatly in different individuals, but is usually over 200. 
The straight, or only slightly curved, shaft is heavily fringed, and borne 
on the customary spherical base. Their dimensions are : 
Length, from 65 » to 175 p. 
Diameter, “ 15 4“ 18 p. 
A transverse section of the shaft near its base has the peculiar shape 
shown in Figure 35 (Plate 7). This modification of the form of its 
wall, found also in the otocyst hairs of the lobster, doubtless renders 
the shaft more rigid than if it were a simple hollow cylinder. 
