GENERAL CHARACTERS. 509 



many types appear to have assumed particular specialisations, 

 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 most Teleostomes 

 the same structure is present, but though occasionally of some 

 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 (dermotrichid). 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. 



TJie skin usually bears numerous scales, mainly or wholly due 

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

 produce enamel. They vary greatly in form 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 t usually aggregated on the head, and 

 arranged in one or more " lateral lines "' along the trunk. 

 There are no muscular elements in the dermis. The muscle 

 segments or myotomes persist as such in adult life. 



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

 lies in front of t/ie genital and urinary aperture, or apertures. 

 Except in Dipnoi, the nostrils do not communicate ivith the 

 mouth, and are exclusively olfactory organs. There is no 

 tympanic cavity or tympanum. 



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, 



