L72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ish or ovoid cavity, lined with chitin; the opening, if any, is always 
dorsal, and varies greatly in size. It is found in the basal segment of 
the first antenne of all decapods, and in the endopod of the sixth or last 
abdominal appendage of the schizopods. The sac is closed in the Bra- 
chiura and Schizopoda, but open in most Macrura. The otocysts of 
Crangon, Palemon, Hippolyte, Mysis, and Carcinus mzenas are described 
in more detail, but no good figures or sections are given. 
Auditory Hairs or bristles. Hensen gives the first and only good 
description of these. They differ from common tactile hairs in that the 
hair shaft is not directly connected with the wall of the sac, but a thin 
chitinous membrane intervenes, forming a small hollow sphere. It is 
this ‘‘ spherical membrane ” which allows the great freedom of move- 
ment necessary for the shaft in its response to sound vibrations. A 
peculiar process, the “lingula,” projects from the inner wall of the base 
of the shaft into the spherical membrane, and to this the nerve fibre is 
attached. The hair shaft is generally plumed, as in tactile hairs, with 
delicate chitinous filaments. 
In A. marinus the hairs are plumed and are nearly one millimetre in 
length. They are here very numerous, 468 having been counted in one 
case, and are arranged on the floor of the otocyst in four parallel semi- 
circular rows. 
A. fluviatalis has a much smaller number of hairs, but the same general 
arrangement ; Crangon, a row of only seven or eight ; these are more 
attenuate than in either of the above forms, but are 0.75 mm. in length. 
Palemon antennarius has about 40 hairs, arranged in a half-oval or 
horseshoe shape, the break in the oval being posterior. The hairs them- 
selves are peculiar in having their shafts bent at a sharp angle. The 
portion of the shaft above the bend is much longer and more attenuate 
than the basal part, and is also heavily plumed. These plumed ends 
project toward the centre of the horseshoe, and intertwine. Their length 
is about 100 u and their greatest diameter 3.84. The hairs of Hippolyte 
and Mysis strongly resemble those of Paleemon, but they are embedded 
in the single otolith and are therefore unplumed. 
Carcinus menas has about three hundred auditory hairs. They are 
grouped into three classes: — 1. Hook hairs (Hakenhaare): the shaft 
hooked and with a plumed tip, about thirty in number, 50 uw long, similar 
to the otolith hairs of Macrura. 2. Thread hairs (Fadenhaare) : long, 
filamentous, plumed at very tip, a single row of about 46, each 338 uw 
long, 3 in diameter. 3. Tuft hairs (Gruppenhaare): short, blunt, and 
unplumed, about 200 in number, occurring in a single large group. 
