GENERAL CHARACTERS. 481 



such as the lungs of Dipnoi, which point forward to the 

 epoch-making transition from water to dry land. 



General Characters. 



Fishes are aquatic Vertebrates, breathing by gills, — vascular 

 outgrowths of the pharynx, bordering gill-clefts and supported 

 by gill-arches. In Dipnoi a single or double outgrowth from 

 the gut — the air- or swim-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 use in respiration, is typically 

 hydrostatic. 



Two pairs of non-digitate limbs, i.e. in the form of fins, 

 are usually present, and there are also unpaired median 

 fins, supported by dermal fin-rays. There are two great types 

 of paired fin. In 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 on one side from several basal pieces, and 

 there is no median axis. 



The skin usually bears numerous scales, in great part due 

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

 produce enamel. They vary greatly inform 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 more " lateral lines" along the trunk. 

 There are no muscular elements in the dermis. 



In many the gut ends in a cloaca, in others a distinct anus 

 lies in front of the genital and urinary aperture, or apertures. 



The heart is two-chambered, and contains only venous 

 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, and a ventricle which drives this through a ventral 

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

 head and by a dorsal aorta to the body. In addition to the 



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