PKENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 201 



shaped. The " mediau " 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 chitiu, 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 transvei'se 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. 



c. 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 diff"erent 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 /i to 175 /a. 

 Diameter, " 15 /x. " 18 fj.. 



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, f )und also in the otocyst hairs of the lobster, doubtless renders 

 the shaft more rigid than if it were a simple hollow cylinder. 



