370 Vertebrata. 



scales;* on the head they are partly enclosed by special, tubular, 

 membrane bones, of which there is, for instance, usually a series 

 below the eye (Fig. 302 s), partly surrounded by the ordinary bones 

 of the head. 



The eyes of Fish are, as a rule, relatively large. The lens 

 is spherical. Movable eyelids are absent ; but the eye is often 

 surrounded by a low circular ridge of skin, and in some species by 

 larger, but immovable folds; in the Mackrel and the Herring, for 

 instance, there is a transparent fold in .front of, and behind, the eye, 

 which partly covers it. 



The scelerotic usually consists of an outer connective tissue layer, and 

 an inner cartilaginous layer, varying in tiiickness (very thick, e.g., in the 

 Sturgeon) ; in the Teleostei the cartilage, in the vicinity of the cornea, is partly 

 replaced by two bony plates, which sometimes attain a considerable importance, 

 and may unite to form a ring. The choroid consists of several layers : 

 usually there is an external lustrous, tunica argentea, a thin coat of con- 

 nective tissue, with an abundant deposit of crystals ; the tapetum lucidwm, a 

 membrane with a metallic lustre, whose cells are filled with crystals, occurs in 

 Selachii, and Ohondrostei, though it is absent from other forms. In the Teleostei 

 there is usually a so-caUed choroid gland in the choroid coat : it is a large 

 horseshoe-shaped rete mirabile near the optic nerve ; this group usually possesses 

 also a processus falciformis, a low fold of the choroid, which runs along the 

 inner side of the optic bidb, from the entrance of the optic nerve to the lens. In 

 certain SqualidsB there is a nictitating membrane, which can be drawn 

 over the eye by a special muscle. 



The auditory organ is only represented by the membranous 

 labyrinth, which is enclosed in the lateral wall of the skull ; within, 

 towards the cranial capsule, the labyrinth is often not completely 

 surrounded by cartilage or bone, but is simply separated from the 

 brain-cavity by connective tissue. In the Selachians, the cavity 

 of the labyrinth opens to the exterior at the surface of the head 

 by a canal, the ductus endolymphatieus ; in others this canal is 

 present, but closed at its outer end. In the Teleostei and bony 

 Ganoids, a large flattened otolith is present in the sacculus, a 

 smaller one in the evagination from the sacculus, and yet a third 

 in the anterior portion of the utriculus. In some Fish these are 

 replaced by bundles of delicate crystals, or by rounded bodies. 



The labyrinth is in a reduced condition in the Cyclostomes, having only 

 one or two semicircular canals, unlike all other Yertebrata. 



The buccal cavity is usually provided with teeth, which in 

 Selachians,! are situated upon the palato-quadrate .and mandibular 

 cartilages; in the Holostei and Teleostei upon a number of different 

 bones : on the premaxilla, maxilla, and mandible, the palatine and 



* The meaning of this is, of course, that the scales in question have developed 

 round the tube after it was formed. 



t Besides the well-developed teeth of the jaws, the Selachians often possess 

 numerous minute teeth on other parts of the waU of the mouth, on its roof and 

 floor, and on the gill bars. 



