PISCES. 
and prominency of these folds there is great 
variety. Inthe Lump-fish ( Cyclopterus ) they 
are hardly perceptible; in the Perch there are 
only sixteen in each nasal sac, and in the 
Turbot twenty-four, whilst in the Conger or 
the Eel their number is prodigious, seeing that 
they extend along the entire length of the long 
tubular nostril. The rays themselves divide 
into secondary folds in the Sturgeon, and per- 
haps in other species ; in short, various modes 
of plication are adopted in different races, but 
the object obtained is the same in all cases, 
namely, an extension of the surface of the 
olfactory membrane. This surface exhibits nu- 
merous delicate vessels, and secretes an abun- 
dant mucosity which lubricates its interior. 
The olfactory nerve, at its commencement 
from the anterior tubercles of the brain, is 
sometimes single, sometimes double, and some- 
times divided into many filaments of variable 
number, length, and thickness in different ge- 
nera, which pass to the posterior or convex 
aspect of the olfactory sacculus. In its course 
and distribution differences are likewise observ- 
able. Thus, in some genera, as the Tetradons, 
it is exceedingly slender; in others, as in the 
Cod (fig. 529), it is likewise of great tenuity, but 
double or triple. The Rays and Sharks have 
it thick and single, and in these races it is 
_ sometimes so short as absolutely to appear 
merely an appendage of the brain. In the 
Tunny likewise it is simple throughout its 
whole length. In the Perch, about the middle 
of its course it divides into two, and its divi- 
sions become multiplied as it approaches the 
nose. In the Conger and Eel it is divided 
almost from its origin into two large trunks, 
each of which gives off successively a great 
number of branches, which subdivide into 
ramuscules to be distributed to all the lamella 
of their long nostril. 
In many genera of Fishes the olfactory nerve, 
at the point where it reaches the nasal cavity, 
dilates into a ganglion, as may be seen in the 
Cod-fish, the Carp,and the Cyprinide generally ; 
and, lastly, the terminal olfactory filaments 
penetrate into all the folds of the pituitary 
membrane, and terminate at their free margins. 
It does not appear, at least in the osseous 
Fishes, that the coverings of the nasal cavities 
or that their openings have any muscles calcu- 
lated to contract or to expand them. 
Eye.—The eye-ball of Fishes presents many 
peculiarities of structure which are rendered 
necessary by their habits for the purpose of re- 
taining the flattened figure of the cornea, and of 
meeting other circumstances of the condition 
under which aquatic vision has to be performed. 
The sclerotic coat which gives shape to the 
entire eye-ball is a dense and fibrous invest- 
ment enclosing the whole eye, except ante- 
riorly, where a space is left for the transparent 
cornea. Its thickness varies in different parts 
to a greater extent than in any other class of 
vertebrate animals, being generally greatest at 
the posterior part of the eye, so as to preserve 
the cup-shaped form of the eye. In the Stur- 
geon, for example, its thickness in this region is 
prodigious, and in the Cod-fish and Shark the 
999 
same circumstance is remarkable, although ina 
less degree. Still further to secure the requisite 
form of the eye strong plates of cartilage are 
very frequently developed in the substance of 
the sclerotic, generally at the back of the eye, 
but sometimes round the cornea likewise, 
which in the larger Fishes occasionally become 
ossified, of which a notable example is met 
with in the Sword-fish ( Xiphias ), where the 
ossified portion of the sclerotic forms a bony 
cup of a spherical form surrounding the entire 
globe of the eye, except opposite the cornea, 
and where the aperture is left for the entrance 
of the optic nerve. 
In the Rays and Sharks among the Chon- 
dropterygii, the sclerotic, which is of a cartila- 
ginous texture, presents another peculiarity in 
the presence of a prominent tubercle, which 
projects externally to be moveably articulated 
with a pedicle of cartilage derived from the 
back of the orbit, which thus forms a pivot or 
centre for the movements of the eyeball. The 
proper cornea is an exceedingly thin laminated 
membrane, filling up the anterior opening of 
the sclerotic; its thickness, however, is consi- 
derably increased by the external integument, 
which passes over it externally under the name 
of membrana conjunctiva: in some species 
indeed, as Cecilia and Gastrobranchus, such 
is the opacity of this tegumentary membrane 
that all vision is precluded. Immediately be- 
neath the sclerotic there is generally a large 
quantity of fatty cellular membrane; this is, 
however, sometimes wanting, but occasionally, 
as for example in the Moon-fish ( Orthagoriscus 
Mola ), its thickness is very considerable. 
On removing this cellular investment a deli- 
cate membrane presents itself, of a brilliant 
metallic lustre (membrana argentea), which 
indeed from its softness resembles rather a 
layer of pigment than a true tunic of the eye- 
ball. It is this layer which spreads anteriorly 
over the front of the iris, giving it the metallic 
brilliancy for which in Fishes it is so remark- 
able. 
The iris itself is formed as in other Verte- 
brata, but the pupil generally remains fixed 
and motionless; the most remarkable pecu- 
liarities noticeable in this part of the eye having 
reference to the shape of the pupil, which is 
very various in its form. Thus in the Grey 
Shark ( Galeus communis ) it is quadrangular ; 
in the Rays and Pleuronectide the pupillary 
aperture is closed by a kind of palmate mem- 
brane, which hangs down like a curtain from 
its upper border; while in one singular case, 
the Anableps, there is a double pupil as well 
as a double cornea, although in all other parti- 
culars the structure of the eye agrees with that 
of ordinary Fishes. 
The choroid of Fishes presents no peculiarity 
of structure worthy of notice; it is very vascu- 
lar and deeply stained with black or dark- 
coloured pigment. As in the higher animals, 
it is separable into two layers: the outer or 
true choroid, which is properly the vascular 
layer, is of considerable thickness, while the 
inner layer forms the tunica Ruyschiana. This 
latter tunic, as it approaches the margin of the 
