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

BOOK 2

is not divided into thorax and abdomen cavities like ours; it is one large cavity. Moreover, this cavity is filled with blood. It is not a special cavity, as our bodycavities are; it is as if the veins of a vertebrate had swollen enormously and united together, squeezing the true bodycavity (and, incidentally, the whole lymphatic system) out of existence. Further, the colour of the blood is different. Instead of our red oxygen-carrying pigment hemoglobin, the lobster has a pale-blue pigment called hemocyanin, with copper in it instead of iron, and instead of being contained in special circulating cells this pigment is diffused, dissolved in the blood-plasma.

Within this vast blood-space his various organs are disposed, and, as a glance at Figs. 5 and 78 will reveal, the manner of their arrangement is quite different from that ofavertebrate. The lobster’s central nervous system runs along his belly; his heart is in the middle of his back; his kidneys and bladders lie in his head and open at the bases of the antennz a little way in front of his mouth ; his or her testes or ovaries (as the case may be) are placed between the heart and the intestine.

As we look more closely into the working of the lobster’s organs we are impressed ever more clearly by his pervading unlikeness to ourselves. In illustration of this point let us follow the course of his food. Our specimen, we may note, eats anything that he can pick up, with a strong preference for animal food—such as smaller crustaceans, molluscs, and worms, or any kind of carrion.

The lobster has no mouth comparable to ours, with lips, tongue, teeth, salivary glands, and so on ; the very thorough chewing that his food undergoes is done partly by the jointed limbs and partly in the so-called stomach. The food is generally seized by the large claws and passed by them to the first or second pair of walking legs ; these in their turn hand it to the last pair of mouthappendages, the third maxillipedes, which tear it to shreds with their rows of sharp teeth. Thence it is handed in turn to the second maxillipedes, the first maxillipedes, the second maxille and first maxilla, all appendages which do but little chewing of the food, and finally it comes to a pair of powerful cutting and grinding jaws, the mandibles. So the food undergoes a thorough mastication, and only when it has been finely minced by this living canteen of appendages is it allowed to enter the mouth.

From the mouth a short gullet leads to the stomach. ‘This stomach is not only a

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THE SCIENCE OF LIFE

CHAPTER 2

digesting bag, as ours is; it is a second mincing machine. It is provided with a beautiful but complicated set of muscles and hard levers of chitin, the gastric mill, which bring about the gnashing together of three powerful teeth. By these teeth the food is reduced to pulp, and at the same time it is acted upon by a digestive juice which oozes forward from the so-called liver.

And here let us interpolate a warning about the names applied to these various organs. ‘That part of the lobster’s body which is called his abdomen is obviously unlike our abdomens; it is differently made, and it contains different structures. Nevertheless, the pioneers of comparative anatomy, men with nimble hands and clumsy tongues, could think of no new word to attach to this highly characteristic structure. They called it abdomen, and abdomen it has remained to this day. Similarly with the lobster’s inside. His stomach does not correspond to our stomachs, and his liver—so called because it is a large gland communicating by means of a narrow tube with the beginning of his intestineis utterly distinct, both in its minute structure and in its working, from our own. When we compared a fish to a mammal we found that both organisms had on the whole the same set of organs, working on the whole in the same way. Wecan speak, without any fear of misleading ourselves, of the liver or stomach or spleen of a fish, because these organs in a fish are essentially the same as our own. Right down to Amphioxus there is a liver. But a lobster is put together in a different way. He has no spleen. His ‘“‘ stomach” is differently made from ours and does different things. He has no organ that corresponds either in structure or working to our livers, and we have no organ corresponding to his. It would be clearest and best to invent a new name for every organ in the lobster’s body, but unhappily this has not been done; we shall have here to follow the general usage and apply the old terms, remembering, however, as we do so that every one of them is a misleading misfit, borrowed from an alien organism.

The lobster’s ‘‘ liver’? is an enormous, tubular gland, manufacturing every one of the digestive ferments that have yet been discovered in the animal: in this respect it corresponds to our salivary, gastric, intestinal, and pancreatic glands combined. It also stores glycogen and fat, resembling our liver in this respect at least. But besides these substances it differs from all our organs