PEENTISS : THE OTOCYST OF DECAPOD CEUSTACEA. 175 



Claus ('75, '91) agrees with Vom Rath as to the nerve ending, but 

 maintains that there is only one ganglion cell sending its process through 

 a growp of matrix cells into the hair. A misundei'standing as to the 

 relations of the ganglion and matrix cells forms the basis of several con- 

 troversial papers. 



Retzius ('90, '92, '95) concludes in his last paper ('95) that there may 

 be several ganglion cells to a single sensory hair. The number may 

 indeed vary from one to many. He was unable by any method to trace 

 the peripheral nerve fibres further than the base of the hairs. Nerve 

 endings, which he described in his first paper ('90) as extending into the 

 hair shaft, he afterwards ('92) frankly acknowledges to be artifacts. 



Bethe ('95"), in his admirable little paper on the otocysts of Mysis, 

 clears up by modern methods many points, and corrects some of 

 Hensen's erroneous descriptions. The sac in Mysis is ellipsoidal, and 

 pointed posteriorly, while from its floor rises a sensory cushion bearing 

 the hairs. This cushion is tilted outwards and ventralwards 45°, the 

 right and left cushions thus being pei-pendicular to each other. The sac 

 is open, not closed as described by Hensen ; the narrow aperture is con- 

 cealed by the overlapping walls of the otocyst. Borne on the sensory 

 hairs is the large otolith, oval as seen from above, kidney-shaped 

 in side view; its greatest diameter 0.3 mm., the other dimensions 

 being 0.25 mm. and 0.15 mm. It is composed of a more or less 

 organic core, about which concentric layers of calcium fluoride are 

 deposited. The tips of the sensory hairs are embedded in this inor- 

 ganic layer, and penetrate to the core of the otolith. The layers of 

 calcium fluoride are probably deposited from the sea water. The sixty 

 sensory hairs are arranged in a single row, so as to form two thirds of 

 a circle, the break in the line being posterior and toward the median 

 plane of the animal. At one end of the curve five hairs are grouped 

 together, and at the other end there is an irregular double row. 

 Though much like the auditory hairs of Paleemon, their tips, em- 

 bedded in the otolith, are unplumed. Only one ganglion cell to a hair 

 was found, sending a distal process into the base of each shaft, A 

 double row of matrix cells lies just beneath the single row of hairs, 

 aud could easily be mistaken for ganglion cells. Yom Rath may have 

 made this mistake, thus getting a multiganglion-celled condition for each 

 hair. 



The otocyst begins to develop before the appendage is fully formed. 

 An invagination of the dorsal ectoderm takes place, producing a shallow 

 depression ; this enlarges while the opening gradually closes. Certain 



