530 PISCES FISHES. 



the plasticity with which 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 Teleo- 

 stomes 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 (dermotrichia). There are two 

 chief types of paired fin, but no hint of the pentadactyl type of 

 higher Vertebrates. In Dipnoi, and in some extinct forms, 

 the fin has a median 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, 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 electric fishes, and rudimentary in eels and 

 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. 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 the genital and urinary aperture, or apertures. 

 The nostrils are paired, and do not communicate with the 

 mouth by posterior nares ; they are exclusively olfactory 

 organs. There is no tympanic cavity or tympanum, or ear- 

 ossicles. 



The heart is two-chambered, and contains only venous 



