AND DISTRIBUTION OF THE FIROLIDÆ. 263 
communication (s); while from its sides and posteriorly it gives filaments (v) to the 
tunic and muscular coat of and near the tail, and large twigs to the caudal muscles. 
The minute structure of the nervous system is very simple, and consists of two ele- 
ments, ganglionic cells and nerve matter, enclosed in a sheath, and readily distinguished 
by their faint yellow tinge. The sheaths, which originate in the fibrous envelope of the 
ganglia (Plate XLIII. figs. 7, 8), are from one-fourth to one-fifth or one-sixth of the 
diameter of the nerve, and contain the nerve proper, transparent and structureless. 
Their ultimate ramifications and mode of termination in the tissues are not easily demon- 
strated. Nor is it possible to distinguish between the sensory and motor element on the 
one hand and the excito-motor on the other, the difference between the two, if appre- 
ciable at all, being still little understood, while the nervous system of such minute 
animals is too delicate to enable us to determine the relative supply to and exact distri- 
bution of either in the various organs. On the other hand the ganglia consist of a mass 
of rounded nucleated cells, comparable to the cortical part of the human brain, the whole 
enveloped in a stout fibrous coat which preserves their form. Their cellular nature is 
most readily demonstrated in the cerebral and pedal, which are both largest and most 
easily brought into view. This cellular element is sometimes prolonged into the at first 
funnel-shaped sheath of the larger nerves (Plate XLIII. figs. 7, 8). The function of 
these ganglia doubtless differs. Thus it is probable that the special senses, viz. sight, 
hearing, and taste, are directly supplied from the cephalie, which also furnishes a large 
portion, if not the whole, of the nerves of ordinary sensation, either directly, or indirectly 
through the intercommunicating cords and other nerve-centres; indeed, from the pro- 
minent place which it occupies in the nervous system of the Firolidæ and Heteropoda 
generally, and the nature of the organs it endows, this ganglion may fairly be regarded 
as the equivalent of the anterior or cephalic pair of the Articulata, and representative 
of the sentient brain of the higher animals; while the other three are probably the 
principal, if not sole source, of motor nerves for the different parts near which they re- 
spectively lie,—the pedal being the largest, in correspondence with the development and 
activity of the swimming-fin; and the branchial or posterior terminal, which supplies 
the tail, as well as the alimentary and reproductive organs, appears to bear a marked 
relation to the size of the steering-apparatus. 
Organs of Special Sense. 
Besides taste and touch, both present in the true Gasteropods and doubtless existing 
in fair development, as they do, though perhaps in a rudimentary state, in more elemen- 
tary forms of life, we find two other senses in the Firola, with well-defined appropriate 
_ organs, viz. hearing and seeing. Olfactory nerves cannot be detected, and the perception 
of smell is doubtful. Bug xo nee ^ 
The eyes (Plate XLIII. figs. 1, 8), very perfect of their kind, indicating a corresponding 
development of the sense of sight, are placed about the anterior third or fourth 
of the dorsal surface, and a little behind and nearer the median line than the ten- 
tacles. Though non-pediculated and fixed, they jut slightly outward and forward (fig. 8) 
with a certain projection which evidently corresponds to the eye-pedicles of the pulmo- 
VOL. XXVII. 2 N 
