278 THE EYE IN EVOLUTION 



As in all Vertebrates below Mammals the decussation of the optic 

 nerve fibres at the chiasma is total. ^ An area centralis, exceptional in 

 Selachians, is commonly seen in Teleosteans and in a number of 

 particularly agile littoral types of this class a fovea is present. 



The ocular movements in Fishes are in general restricted, reflex 

 and primitive, and the extra-ocular muscles are essentially designed 

 to subserve rotations of the eyes compensatory to movements of the 

 body ; with few exceptions ^ fixation is attained, not by movements 

 of the eyes but of the body in swimming. The muscles are therefore 

 designed to subserve merely the simple rotations required by the 

 postural mechanism ; the recti form a cone arising from the apex of 

 the orbit, and the obliques, subserving simple wheel-rotations, arise 

 anteriorly and remain on a plane anterior to the recti (Fig. 293). 



The super-class of Fishes includes an enormous number of forms, many of 

 them long since extinct ; the extant types may be divided into two main classes^ : 



(a) CHONDRiCHTHYES ()(6v8pos. Cartilage ; l^dvs, a fish) (or elasmobranchs 

 — eAaa/i,o9, a metal plate ; branchia, a gill ; so called because of their lamelliform 

 gills) with a cartilaginous skeleton, and 



{b) OSTEICHTHYES (dcTTeov, bone ; l^Ovs, a fish) with a more or less ossified 

 skeleton. 



CHONDRICHTHYES (or cartilaginous fishes) are represented today only by 

 two sub-classes — the selachii (CTeAa;;^os', a cartilaginous fish) which include the 

 families of sharks and skates or rays, and the holocephali (oAo?, whole ; 

 K€<f)C/.Xrj, a head), such as the Chimcera. 



OSTEICHTHYES * (or bony fishes) form a much more heterogeneous class. 

 With the exception of the relatively inodern Teleosteans, most types are largely 

 extinct and are now represented by few species, but all of them flourished in 

 large numbers in ancient times. The class is conveniently divided into 6 groups. 



The DIPNOI ( St?, twice ; 771^017, breath) (lung- or mud-fishes) are a very 

 ancient form abundantly represented by fossils in the Mesozoic beds throughout 

 the world but today found sporadically as three genera only in Eastern Australia, 

 in the marshes of Africa and the swamps of the Amazon basin. Their skeleton 

 is largely cartilaginous and their name is derived from their double method of 

 breathing, for their air-bladder is developed to form a breathing lung. 



The ccELACANTHiNi are represented today only by one living species — 

 Latimeria, a fish thought to have disappeared 80,000,000 years ago but recently 

 discovered in the coastal seas of south-east Africa. The Coelacanths are 

 characterized by a skeleton, part bone, part cartilage, basal skeletal supports 

 formed by a solid projecting lobe on which the fringe-like pectoral Und pelvic 

 fins are set. 



The CHONDROSTEi {-^ovhfjos, cartilage ; oGrdov bone) — fishes with a cartila- 

 ginous internal skeleton — are represented today only by a few species of 

 sturgeons and the polypterini (tvoAl'?, many; irrepov, awing) which have a series 

 of finlets instead of a dorsal fin. The latter survive as two types found in African 

 rivers {Polypterus or bichir, and C alamo ichthys). The skeleton is very bony, 

 and till bilobed air-bladder, the duct of which opens ventrally into the pharynx, 



^ i".;o, however, some Reptiles which form an exception (p. 392). 

 2 p. «93. » p. 234. 



* E; 'uding Dipnoi and Teleostei this large class used to be known as ganoids 

 iyavos, li. ') on account of their ganoin-coated scales. 



