PISCES. 



999 



and prominency of these folds there is great 

 variety. In the Lump-fish (Ci/clopterus) 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 (Jig. 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 appeal- 

 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 lamellae 

 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 Cyprinidoe 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 



same circumstance is remarkable, although in a 

 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 ( Orthagorisctis 

 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 Pleuronectidae 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 Rui/schiana. This 

 latter tunic, as it approaches the margin of the 



