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 ave aquatic Vertebrates, breathing by gills,—vascular 
outgrowths of the pharynx, bordering gill-clefts and supported 
by gill-arches. In Dipnot 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 ts present, but though occasionally of 
some use in respiration, ts typically hydrostatic. 
Two pairs of non-digitate limbs, i.e. in the form of fins, 
ave 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 ts 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 Dipnot 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 
