476 FISHES. 



been followed by a yet richer predominance of the modern 

 Bony Fishes ; and, furthermore, in the plasticity with which 

 many types appear to have assumed particular specialisations, 

 such as that evolution of lungs which, in the double breath- 

 ing Dipnoi, prophesies the epoch-making transition from 

 water to terra firma. 



General Characters. — Fishes are aquatic Vertebrates, 

 breathing by gills attached to bony or gristly arches on the 

 sides of the pharyngeal gill clefts. In Dipnoi, a single or 

 double outgrowth from the gut — the air- or switn-bladder — 

 functions as a lung, air being inspired at the surface of the 

 water. In Ganoids and in most Teleosteans the same 

 structure is present, but though occasionally of some slight 

 usefulness in respiration, usually serves as a hydrostatic 

 organ. 



Tivo pairs of limbs, in the form offifis, are usually prese7it, 

 and there are also unpaired median fins, supported by fin- 

 rays. There are two great types of paired fin. Ii Dipnoi, 

 and in some extinct forms, the fin has a central segmented 

 axis, which (e.g., Ceratodus) bears on each side a series of 

 radial pieces. In other fishes the radials diverge outwards 

 from several basal pieces, and there is no median axis. 



The skin usually bears niunerous scales, in great part due 

 to the dermis, but covered by a layer of epidermis, which ?nay 

 produce enamel. They vary greatly in forjn and texture, are 

 suppressed in eels and electric fishes, and rudimentary in some 

 other forms. Numerous glandular cells occur in the skin, 

 but these are not compacted into multicellular glands, except 

 in Dipnoi and a few poisonous fishes. The skin also bears 

 sensory structures, usually aggregated on the head, and 

 arranged in one or j?iore " lateral lines " along the trunk. 

 There are ?io musctilar elements i?t the dermis or cutis. 



In many the gut ends in a cloaca, or a distitict anus may 

 lie in front of the geftital atzd urinary aperture, or apertures. 



The heart is two-chambered, and contains onlv ve?ious 

 blood, except in the Dipnoi, where it shows hints of becoming 

 three-chambered and receives pure blood from the lungs as 

 well as impure blood from the body. Apart from the Dipnoi, 

 the heart has a single auricle, receiving impure blood from 

 the body, a?id a ventricle ivhich drives this through a ventral 

 aorta to the gills, ivhence the purified blood flows to the 



