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

BOOK 2

as balancing vanes, partly like the ailerons of an aeroplane, to keep the fish the right way up, and partly like elevating-planes to steer it upwards or downwards through the water. A mammal, on the other hand, living in a medium with less resistance than water, has no need of accurate stream-lining, and can afford the luxury of a neck; at the same time he cannot propel himself by wagging his tail, and so his limbs must form “Tong supporting and propulsive levers. The difference in respiratory mechanism has a similar rationale. A dogfish breathes by gulping a current of water into his mouth and out through the gill-slits. As the water passes over his gills it yields oxygen to the blood in them and washes away the carbon dioxide. A mammal could not do this; if he passed a continual current of air over his respiratory membranes they would dry up, and moisture plays an essential part in the exchange of gases. He breathes by drawing air into those hollow bags, his lungs, and blowing it out again. In the dogfish there are no lungs ; we do not even find a homologue here, though many other fishes have it in the form of the swimming bladder. We may note that a dogfish could not inhale and exhale water as the mammal does air, because the inertia of water is too great. Running along the side of the dogfish’s body is the lateral line, seen as a white streak in the drawing. This is a sense-organ, probably recording changes in pressure ; owing to its structure it would not work in air, and we do not find it in mammals.

So far as outward and visible structure is concerned, a dogfish is like a mammal, but twisted about and modified because of the necessities of an aquatic existence. Or we may put it round the other way and say the mammal is after the dogfish pattern but adapted to the air. If we turn to internal anatomy we find that the correspondence of lay-out still holds. The dogfish, like the mammal, has a skull and a backbone made of jointed vertebre ; the former houses the brain and the latter has a canal running through it which contains and protects the spinal cord. He has a heart in a position corresponding to our hearts, but as he has no lungs he has no double circulation ; the dogfish heart, like the heart of a human embryo, is a single pump and drives all the blood forward via the gills to the rest of the body. He has a stomach lying rather to the left of his abdominal cavity and kidneys at the back ; he has a liver and portal veins, a spleen, thyroid, pineal and_ pituitary

108

THE SCIENCE OF LIFE

GHAPTER 1

glands, cerebrum and cerebellum, testes or ovaries. ‘The homology of his organs can be traced even in minute details, in the course and branching of his nerves and arteries. Upon point after point the differences between a dogfish and ourselves are modifications of a plan common to both. And in many cases where the homologies of the two adults are not apparent a study of the human embryoshows how the twocorrespond.

The position might be summarized by saying that if we had a model dogfish made of plasticine we could twist and stretch and bend it bit by bit, increasing the quantity at this point perhaps and diminishing it at that, but adding nothing essentially new, until we had a very passable model mammal—a modifying process that is crudely reproduced in the stages of our own embryonic development. If we examined any other vertebrate—a frog, say, or a tortoise, or a sparrow—we should find a similar agreement in plan. They all possess practically the same set of organs, put together in the same way. On the other hand this does not apply to a great multitude of other living animals. A lobster, for example, has organs that do not correspond at all to ours, and the whole architecture of his body is distinct ; the same is true of a starfish, or a tapeworm, or an oyster. A plasticine model of any of these creatures would have to be broken down to nearly nothing before you could begin to worry it into the form of a vertebrate. You might perhaps retain mouth and anus, but that would be almost all.

Such, then, is the import of “ phylum.” If we have a thorough knowledge of the anatomy of one particular kind of animal we are able to find our way about any other kind of animal belonging to that same phylum with only moderate difficulty. For example, knowing the plan on which a mammal works, we know roughly what organs to expect in a bird or a fish and what part of the body they are in. But if our knowledge is confined to that one kind of animal we are hopelessly at sea if we try to dissect a member of a different phylumif we tried, for example, to find organs corresponding to the organs of mammals in the inside of a lobster.

In’ reply to the question, What are the characteristic features of Vertebrates, as distinct from other animals? a trained zoologist could reel off a string of anatomical points, particularly if he was a_ teacher or a student about to undergo examination. We shall discuss some of these points later ;