ORGAN OF SIGHT IN FISHES. 331 



and Crocodilia, the external opening to the nasal organ in the skull 

 is single and median, situated at or near the end of the muzzle. 

 But in the Chelonia the nostrils are distinct, although approximate, 

 on the integument : in Trionyx and Chelys they are tubular, con- 

 tinued along a short proboscidiform production of the integument. 

 The septum narium is gristly. In the Turtle ( Chelone) the nasal 

 cavity suddenly expands to contain the turbinal cartilage. The 

 periosteum of the cavity and the pituitary membrane are both 

 coloured by dark pigment, and the latter is thick and vascular. 

 The palatal orifice is median and single, towards the fore part of 

 the roof of the mouth. In the Crocodilia the tegumentary nostril, 

 like the osseous one, is single, crescentic, with the concavity 

 backward, and closed by the fleshy posterior valvular lobe : in the 

 Gavial the tegument surrounding the nostril is thick, abundant, 

 and can be raised from the bone, or erected, to bring the orifice to 

 the surface of the water without exposure of other parts of the 

 head. The nasal cavity is of great length, commencing at the 

 fore part of the muzzle, and terminating beneath the occiput, also 

 by a single aperture, close to which the nasal septum terminates. 

 The anterior third part of the meatus is most expanded : the 

 pituitary membrane is extended upon a bilobed turbinal, partly 

 bony and partly gristly : the meatus also communicates with 

 large cells or sinuses. 



§ 64. Organ of Sight in Fishes. — The organ of sight makes its 

 appearance in the lowest of Fishes, e. g. the Lancelet and Myxine, 

 under as simple a form as in the Leech : a minute tegumentary 

 follicle is coated by dark pigment, which receives the end of a 

 special cerebral nerve. This simple eyespeck, the first mechanism 

 for the appreciation of light, is repeated in the Amblyopsis spelceus, 

 fig. 175, o. Rudimental eyeballs covered by the skin exist in the 

 Apterichthys ccecus : the small, but more complex, eyes of the 

 Lepidosiren, with crystalline and vitreous humours, choroid and 

 sclerotic tunics, are also covered by the skin, but this becomes 

 transparent where it passes over them, and, adhering to the 

 sclerotic, forms a ' cornea.' The eyes of the Eel tribe and the 

 Siluroid Fishes are small : they are of moderate size in the 

 Plagiostomes and Ganoids ; but in most Osseous Fishes the eyes 

 are remarkable for their large size, which becomes enormous in 

 some, e. g. Orthagoriscus, Myripristis, Priacanthus. The eyes 

 are usually placed in orbital cavities, one on each side of the 

 head ; only in the unsymmetrical Flat-fish are they both placed 

 on the same side : in the Stargazer ( Uranoscopus) the eyes are 

 approximated on the upper surface of a nearly cubical head, and 



