172 bulletin: museum of comparative zoology. 



ish or ovoid cavity, lined with chitiu ; the opening, if any, is always 

 dorsal, and varies greatly in size. It is found in the basal segment of 

 the first antennae 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, Palfemon, Hippolyte, Mysis, and Carcinus msenas 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. 



Palaemon 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// and their greatest diameter 3.8 /y. The hairs of Hippolyte 

 and Mysis strongly resemble those of Palsemon, but they are embedded 

 in the single otolith and are therefore unpltmied. 



Carcinus maenas has about three hundred auditory hairs. They are 

 grouped into three classes : — 1. Hook hairs (Hakenhaare) : tlie shaft 

 hooked and with a plumed tip, about thirty in number, 50 /i 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 (i 

 long, 3 |U in diameter. 3. Tuft hairs (Cruppenhaare) : short, blunt, and 

 unpluraed, about 200 in number, occurring in a single large group. 



