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

THE FIRST GREAT PHYLUM:

a variety of names on the counters of our fried-fish shops. His general appearance is shown in the accompanying drawing.

Fig

VERTEBRATES

their position and in the fact that they are supported by skeletal girdles inside the body.

Many of the differences in external appearance between a dogfish and a mammal may be directly correlated with the fact that the former lives in the sea while the latter lives on land. The body of the dogfish

(From the Plymouth Aquarium Guide Book, by courtesy of the Marine Biological Association of the United Kingdom.)

Like an adult mammal, he has a head with eyes and mouth and nostrils that lead into an olfactory cavity ; like an embryo mammal, he has a series of gill-slits on the sides of his throat. The front gill-slit, or spzracle, we may note in passing, is different in structure and position from the rest and different in a very amusing and interesting way. It is small and round and placed just behind the eye, and although it is not used for hearing, it runs very close to the inner ear of the dogfish and then opens into his mouth. In its position, and in the particular nerves and arteries that lie against it, it corresponds to the channel made by our outer and middle ears and Eustachian tubes, and this idea of the correspondence of the two structures is confirmed by the fact that in a mammalian embryo the front gill-slit actually becomes separated from the rest and develops directly into that channel. Biologists call this parallelism homology ; the two structures are ‘“‘ homologous.” And in creatures of the same phylum, but of different orders or classes, we shall be constantly detecting homologies. _ The fins of a dogfish are of two kinds—the unpaired fins, including the fins of his back and tail and the hindmost of the fins on his lower surface, and the paired fins, of which there are two pairs. The unpaired fins do not correspond in structure or position to anything that is found in a mammal, but the paired fins are homologous with our limbs ; they resemble our limbs in

Fig. (B), a chick (C), a rabbit (D), and a man (£) resemble each other very closely ; (magnified).

As development proceeds these creatures arise as divergent modifi-

cations of the primitive plan.

is beautifully stream-lined, so that he may glide swiftly through the water. His tail is a powerful organ of propulsion. His paired fins do not project far from the trunk, for they would offer too much resistance to the water if they did so; they are used

69. Early embryos of a dogfish (A), a lizard

2?

(From “‘ Vertebrate Zoology, by G. R. de Beer, Sidgwick G Jackson.) 107