STRUCT QBE AND DEVELOPMENT OP LOXOSOMA. 271 
tactile prominence on either side of the posterior wall of the 
calyx, situated not far from the level of the ventral surface of 
the stomach (fig. 1, ps.). Each of these sense organs, whose 
structure is described below, bears a tuft of long stiff hairs. 
The tentacles are provided with a rich development of sense- 
cells, supplied by nerves (one for each tentacle) connected 
with the ganglion. Owing to the somewhat varying number of 
the tentacles, the arrangement of their nerves cannot be 
absolutely constant, although certain general features can 
always be made out. The tentacles belonging to the posterior 
half of the lophophore appear to be invariably supplied by two 
pairs of nerves running in the posterior vestibular wall. The 
more median pair arises directly from the ganglion, each of 
the two nerves usually supplying three tentacles in the manner 
indicated in the figure. The outer pair arises from the nerves 
of the posterior tactile prominences, and usually forks so as to 
give rise to two tentacular branches. The origin of the nerves 
belonging to the tentacles of the anterior half of the lopho- 
phore is less easy to determine; but their arrangement, as 
given in fig. 1, is probably not very inaccurate. With the 
exception of the two pairs figured, no nerves were with certainty 
observed to arise directly from the ganglion, 1 although it is 
possible that those supplying the dorsal regions of the body 
may have this origin. At the base of each tentacle the nerve 
swells into a small ganglion (fig. 1, tga.), consisting in most 
cases of about four or five bipolar cells. This ganglion gives 
off one or two nerve-fibrils, passing directly to as many sense- 
cells, and is prolonged into a fine nerve, which passes up the 
axis of the tentacle, the rest of whose sense- cells it supplies. 
These tactile organs are not very numerous, and they occur 
entirely on the outer (unciliated) and lateral portions of the 
tentacle. Each bears a fine stiff hair, or more rarely two or 
three. A single large hair occurs in the middle of the convex 
side of each tentacle near its apex, and when these organs 
■ In the adult, there is no trace of a brain or of circumcesophageal com- 
missures, even the tentacles of the anterior portion of the lophophore being 
undoubtedly supplied with nerves from the subcesophageal ganglion. 
