The science of life : fully illustrated in tone and line and including many diagrams

THE FIRST GREAT PHYLUM: VERTEBRATES

they have one or two; in some species the hollow pituitary pouch of the brain opens into the roof of the mouth. But their most striking peculiarities are in the structure of the mouth itself. They have no jaws and no proper teeth; instead of being a slit that can be open or shut, their mouth is a funnel that can be used as a sucker, and their tongues, armed with horny substitutes for teeth, can be moved like a piston in the mouth-funnel. They nourish themselves by attaching themselves to fishes by means of this sucker and rasping off the flesh with their tongues. ‘They may even bore through to the abdominal cavity of their victim, punching a neat round hole in his body-wall as they do so.

The earliest known fossil vertebrates are certain heavily-armoured fish-like forms more like these Cyclostomes than any other creature. It has been shown that these, too, were jawless, and that they were built in the same sort of way as a lamprey or a hag-fish. Their method of feeding, however, was probably different. Apparently the lampreys and hag-fish are, like the reptiles, the rare survivors of a once dominant aristocracy, but one enormously more ancient than that of the reptile world.

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Semi-Vertebrates

The animals that we have already considered share a common plan of organization. Their most evident distinctive feature is the possession of a backbone, a column of ringlike blocks of bone or gristle jointed together,

running down the back and housing the .

spinal cord. Because of this vertebral column the animals are called vertebrates. The phylum includes ourselves, our domestic animals, and all the largest and most conspicuous wild creatures; it is the most immediately striking and for practical purposes the most important phylum. Since they are less obtrusively important, all the animals that are not vertebrates are often lumped together as the invertebrates, although this negative group, including as it does a number of different phyla, is in reality a much more varied one than the single phylum of vertebrates. It was inevitable that the earliest naturalists should pay more attention to vertebrated animals than to others, and the thorough exploration of the great group of outsiders and the recognition of their variety and systematic importance only followed long after the general principles of vertebrate classification had been recognized. How the inverte-

brata have been at last sorted out into a system of phyla we shall tell in the following chapters, but here we must note certain odd lowly forms which naturalists, after the most careful consideration, have decided to throw back into the vertebrate basket. They are not full vertebrates certainly, but they are put with them as associates, distant cousins, semi-vertebrata.

The lancelet—Amphioxus (sharp at both ends) is its systematic name—is an inconspicuous little marine animal, whitish, translucent, shaped like a flat toothpick, and less than two inches long (Fig. 77). It lives an obscure life, burrowing in sand in shallow water round most coasts, and was considered by its discoverer to be a slug. But subsequent investigation has removed it from the low phylum to which it was assigned and placed it with the vertebrates. If it is not legitimately a vertebrate, it is at least a poor relation.

This little two-inch slip of life has a smooth limbless body. Its mouth is surrounded by tentacles and leads into the pharynx, a perforated, nearly cylindrical structure not unlike a gas-mantle. The fabric of this gas-mantle is covered with cells bearing tiny vibrating hairs or cilia of the same nature as those in our air-passages, and these microscopic hairs are continually working water through the perforations in an outward direction. ‘The space round the gasmantle opens into the sea by a special pore ; thus there is a continual slow current of water in by the mouth, through the slits of the gas-mantle, out by the special pore. This apparatus is not unlike the gill-apparatus of a fish, for here also there are clefts in the side of the throat through which water passes out, and it was for a long time regarded as a similar respiratory device. But it is now known that the chief duty of the apparatus is to collect food. As it passes through this animated sieve the water is filtered; minute organic particles are collected on strands of sticky mucus and passed down to the gullet, which opens at a place corresponding to the blind end of the gas-mantle. On the nutritious matter which is collected in this way the animal subsists.

Amphioxus resembles the vertebrated animals in several important respects. In the first place, it has a tail. In invertebrate animals the anus generally opens at the extreme hinder end of the body, and there is never the muscular backward prolongation of the body-wall, containing the end of the spinal cord, that is found in all embryo vertebrates, and in the great majority of adult

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