ORGAN OF SIGHT IN FISHES. 331 



and CrocodiUa, the external opening to tlie nasal organ in the skull 

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

 -But in the Chelonia the mistrils are distinct, although approximate, 

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

 tinned along a short proboscidiform production of the integument. 

 The sei)tum narium is gristly. In the Turtle (yChelone) the nasal 

 cavity suddenly expands to contain the turbinal cartilage. The 

 y)eriosteum of the cavity and the pituitary membrane are both 

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

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

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

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

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

 Clavial the tegument surrounding the nostril is thick, abundant, 

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

 the surface of the water without exposure of other j)arts of the 

 bead. The nasal cavltv is of g-reat lenii'tli, commencinjr at the 

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

 Ijy 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 ujion a bilobed tiu-binal, partly 

 bony and [lartly gristly : the meatus also communicates with 

 large cells or sinuses. 



§ 64. Or(/an of Sit/ht 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 teginnentaiy 

 Ibllicle 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 AmJAi/opsis spelaus, 

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

 AjiU'i-iciitJu/s ca'ciis : the small, but more complex, e}'es of the 

 Lepidosiren, with crystalline and vitreous humours, choroid and 

 sclerotic tunics, are also covered by the skin, l^ut this Ijecomes 

 transparent where it passes over them, and, adliering 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, Mijripristis, 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 jilaced 

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

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



