The came for the vegetative increase in the numbers of vertebra as we leave the tropical shores may perhaps be found in the reduction of the stress of natural selection. For such icessation of selection) the name panmixia has been suggested by Dr. August Weismann. It should be noticed that the increase in the num ber of vertebra is accompanied, necessarily, by the loss of importance and the reduction of in each individual one. In this the fin-rays usually share, there being a greater number, as a rule, in forms outside the tropics as compared with similar forms on warm shores.
Organs of The organs of nutri tion in the fish are generally homologous with those of higher vertebrates. Some fish feed ex clusively on vegetable food. These have gen erally incisor-like teeth loosely planted in the gums and a greatly elongate alimentary canal.
Most fishes feed on animal food, other fishes„ crustaceans, mollusks, worms, insects and whatever else may be found in the sea. The mouth and teeth are adapted to the kind of food and the varieties in form and armature are even more varied than the forms of the body among fishes.
The mouth is usually capacious. It may have no teeth; it may have teeth on the jaws only, or any bone in its circumference may be armed with them. In most fishes additional teeth of one sort or another may be found on thepharyngeals, in the gullet behind the gills.
The stomach may be U-shaped — an opening at either end—or it may have the form of a blind sac, the two openings close together. Often glands, called pyloric caeca, secreting a digestive fluid, occur at the end of the stomach.
The intestine May be a single tube, or it may be variously complicated, wound in spiral, or as in the shark, having its surface increased by means of a spiral valve within. The liver Fall-bladder and spleen are usually developed in fishes much as in the higher forms.
Fishes lack salivary glands. The tongue is cartilaginous or bony, with few nerves of taste, although taste buds may exist in the barbels, as in the cat-fish. Kidneys lie along the inner edge of the backbone, less specialized than in the higher vertebrates, their ducts leading into a common cloaca. In the lower fish-like forms (lancelets) the structures which serve the pur pose of kidneys bear a close resemblance to similar glands (nephridia) in worms.
Organs of Respiration and Circulation.— In the lowest fish-like forms (lancelets), the heart is reduced to a simple pulsating tube. In the typical fishes it consists mainly of three parts, the auricle, the ventricle and a thickened part of the large artery, known as the bulbs: arteriosus. These parts correspond in a gen eral way to right auricle and right ventricle in the higher vertebrates. The blood reaches the heart from the vein-system of the body. It is passed from auricle to ventricle, then sent through a large artery to the gills to be puri fied. From the gills it is distributed through the main artery or aorta to the different parts of the body, to be again gathered up from the capillaries into the veins.
It is not returned to the heart after purifi cation, as in the higher vertebrates. Its flow is therefore relatively sluggish, and, as in rep tiles and amphibians, its temperature is little above that of the environment. In some of the more primitive fishes the heart is more com plex than in the typical forms, the arterial bulb being provided with a large number of valves.
All the chordate animals, including the tun icates and the Enteropneusta, have the purifica tion of the blood effected through some form of gill-slit. In the lower forms these are merely slits, admitting water to the pharynx, where it comes in contact with thin membranes covering capillaries filled with blood.
In the typical fishes a much more elaborate apparatus is developed. There are usually five long bony arches attached to the cranium above and to the base of the tongue below. Each of these has two rows of slender filaments, the gills. The blood flows in and out of these where it comes in contact with the water. this water is swallowed through the mouth, and is forced out through the gill opening. Behind the true gill-arches is a fifth arch, more or less similar usually without gills, being modified to form, a pair of gullet jaws, the pharyngeals. On the anterior edge of the first gill-arch is a series of projections, sometimes forming a straining apparatus. These are the pharyngeals. A small accessory gill, • the pseudobranchia, is usually developed on the inner sideof the opercle or gill-cover.