Fishes, constituting the class _Pisces_, may be defined as Craniate
Vertebrata, or Chordata, in which the anterior portion of the central
nervous system is expanded into a brain surrounded by an unsegmented
portion of the axial skeleton; which are provided with a heart,
breathing through gills; and in which the limbs, if present, are in the
form of fins, as opposed to the pentadactyle, structure common to the
other Vertebrata. With the exception of a few forms in which lungs are
present in addition to the gills, thus enabling the animal to breathe
atmospheric air for more or less considerable periods (Dipneusti), all
fishes are aquatic throughout their existence.
In addition to the paired limbs, median fins are usually present,
consisting of dermal rays borne by endoskeletal supports, which in the
more primitive forms are strikingly similar in structure to the paired
fins that are assumed to have arisen from the breaking up of a lateral
fold similar to the vertical folds out of which the dorsal, anal and
caudal fins have been evolved. The body is naked, or scaly, or covered
with bony shields or hard spines.
Leaving aside the Ostracophori, which are dealt with in a separate
article, the fishes may be divided into three subclasses--
I. Cyclostomi or Marsipobranchii, with the skull imperfectly developed,
without jaws, with a single nasal aperture, without paired fins, and
with an unpaired fin without dermal rays. Lampreys and hag-fishes.
II. Selachii or Chondropterygii, with the skull well developed but
without membrane bones, with paired nasal apertures, with median and
paired fins, the ventrals bearing prehensile organs (claspers) in the
males. Sharks, skates and chimaeras.
III. Teleostomi, with the skull well developed and with membrane bones,
with paired nasal apertures, primarily with median and paired fins,
including all other fishes. (G. A. B.)