STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 481 
also depends upon whether or not the absence of collaterals 
represents the true condition. This is almost certainly the 
case on account of the ease and frequency with which these 
elements stain, and the readiness with which their branches in 
the brain are demonstrated. It will be remembered, too, that, 
according to Retzius, the branches into which the fibres divide 
in the last abdominal ganglion pass out through the nerves of 
that ganglion, and it therefore seems probable that they serve 
the purpose of putting some organ into direct communication 
with the brain. It may be that these branches go directly to 
the muscles which control the tail fin, so that the steering 
apparatus of the animal is under the direct control of the 
brain. This, at any rate, is the most obvious suggestion. 
It is of course possible that the other elements of the class 
A serve a similar purpose, but the fact that they have collateral 
branches in each ganglion seems to me to be opposed to this 
view. 
The above remarks are intended merely as an indication of 
the nature of the problems suggested by the observations 
recorded in the previous part of the paper, problems which can 
only be finally solved by means of physiological research, for 
which research, however, an accurate knowledge of anatomical 
details is an essential preliminary. 
EXPLANATION OF PLATES 35 and 36, 
Illustrating Mr. Edgar J. Allen’s paper on ‘Some Nerve 
Elements of the Embryonic Lobster.” 
Fic. 1.—Brain and thoracic ganglia of Homarus embryo. Motor ele- 
ments. @s. Gisophagus. st. com. gang. Commissural ganglion from which 
stomatogastric nerves arise. ¢r. dr. Transverse bridge behind csophagus. 
com. (sophageal commissure. ant. x. r, Anterior nerve-root. post. n. r. 
