PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 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 group of matrix cells into the hair. A misunderstanding 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 perpendicular 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 Palemon, 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, 
and could easily be mistaken for ganglion cells. Vom 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 
