The Ichthyopsida

The Ichthyopsida.- 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 other Vertebrata.

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

1. 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.
2. 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- CARDIA. Hck.)
   1. The nasal sac is single, and has a median external aperture. Neither mandibles nor limb arches are developed. (Monorhina, Hck.)
   II. - Marsipobranchii.
   2. There are two nasal sacs with separate apertures. Mandibles and limb arches are developed. (Amphirhina. Hck.)
      1. The nasal passages do not communicate with the cavity of the mouth. There are no lungs, and the heart has but one auricle,
         1. The skull is devoid of membrane bones.
         III. - Elasmobranchii.
         2. Membrane bones are developed in relation with the skull.
            1. The optic nerves fosm a chiasma, and there are several rows of valves in the aortic bulb.
            IV. - Ganoidei.
            2. The optic nerves simply cross, and there is only one row of valves in the aortic bulb.
            V. - Teleostei.
      2. The nasal passages communicate with the oral cavity. There are lungs, and the heart has two auricles.
      VI. - Dipnoi.

I.-The Ichthyopsida

1. Have the epidermic exoskeleton either absent, or very slightly represented.
2. The spinal column may persist as a notochord with a membranous sheath, or it may exhibit various degrees of chondrification or ossification. When the vertebrae are distinct, their centra have no epiphyses.
3. The skull may be incomplete and membranous, more or less cartilaginous, or osseous. When membrane bones are developed in connection with it, there is a large parasphenoid. The basisphenoid is always small, if it be not absent.
4. The occipital condyle may be absent, or single, or double. When there are two occipital condyles they belong to the ex-occipital region, and the basi-occipital region is unossified or very imperfectly ossified.
5. The mandible may be absent, or be represented only by cartilage. If membrane bones are developed in connection with it, there is usually more than one on each side. The articular element may be ossified or not, and may be connected with the skull by the intermediation of a quadrate and a hyomandibular element, or by a single fixed plate of carti lage representing both these and the pterygo-palatine arch. A stapes may be present or absent.
6. The alimentary canal may or may not terminate in a cloaca. When there is no cloaca, the rectum opens in front of the urinary organs.
7. The blood-corpuscles are always nucleated, and the heart mav be tubular, bilocular, or trilocular.
8. There are never fewer than two aortic arches in the adult.
9. Respiration takes place by branchiae during part, or the whole, of life.
10. There is no thoracic diaphragm.
11. The urinary organs are permanent Wolffian bodies.
12. The cerebral hemispheres may be absent, and are never united by a corpus callosum.
13. The embryo has no amnion, and, at most, a rudimentary allantois.
14. There are no mammary glands.