.- Class I. - PISCES.
The class of Fishes contains animals which vary so much
in their grade of organization, and in their higher forms so
closely approach the Amphibia
, that it is difficult to draw up
any definition which shall be at once characteristic and diagnostic
of them. But they are the only vertebrated animals
which possess median fins supported by fin-rays; and in
which the limbs, when present, do not exhibit that division
into brachium, antebrachium, and manus, which is found in all
The presence of the peculiar integmentary organs constituting
what is known as the system of mucous canals and
the organs of the lateral line (supra
, p. 79), is highly
characteristic of Fishes, though these organs cannot be said to exist
in the entire class.
The class Pisces is divisible into the following primary groups
- The notochord extends to the anterior end of the body. There are no
skull, brain, auditory, or renal organs, such as exist in the higher Vertebrata.
The heart is a simple tube, and the liver is saccular. (LEPTO CARNIA. Haeckel.)
I. - Pharyngobranchii.
- The notochord ends behind the pituitary fossa. A skull, brain, auditory,
and renal organs are developed. The heart is divided into auricular and
ventricular chambers. The liver has the ordinary structure. (PACIFY-
- The nasal sac is single, and has a median external aperture. Neither
mandibles nor limb arches are developed. (Monorhina, Hck.)
II. - Marsipobranchii.
- There are two nasal sacs with separate apertures. Mandibles and
limb arches are developed. (Amphirhina. Hck.)
- The nasal passages do not communicate with the cavity of the
mouth. There are no lungs, and the heart has but one auricle,
- The skull is devoid of membrane bones.
III. - Elasmobranchii.
- Membrane bones are developed in relation with the skull.
- The optic nerves fosm a chiasma, and there are several
rows of valves in the aortic bulb.
IV. - Ganoidei.
- The optic nerves simply cross, and there is only one
row of valves in the aortic bulb.
V. - Teleostei.
- The nasal passages communicate with the oral cavity. There
are lungs, and the heart has two auricles.
VI. - Dipnoi.