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

THE SECOND GREAT PHYLUM:

in storing lime ; the lobster, being enclosed in a rigid armouring, has to moult from time to time so that he may grow, and calcium salts required for forming a new shell are stored in readiness in the liver. The gland lacks the most characteristic feature of a vertebrate liver, the portal vein, and is in no sense a scrutineer of blood from the bowels. Lastly, it resembles our intestine, for food actually travels into its ducts and tubules to be absorbed.

From the gastric mill the food is passed into a smaller chamber, provided with an elaborate apparatus of sieves and filters, where it is sorted into two fractions. The coarser particles pass into a long, cylindrical intestine, whence much of their nutritious matter is absorbed and whence their residue is ultimately voided. This fraction is of minor importance. ‘The digestive juice, on the other hand, carrying most of the food materials either in solution or in suspension as fine particles, is sucked back into the tubules of the liver, and here the greater part of the absorption of food takes place.

No need to stress the profound dissimilarity between these digestive arrangements and our own. A comparison of any other system of organs would give the same results. We should find just the same distinctiveness throughout. In the disposition and dynamics of the circulation, in the kidneys, in the astonishing compound eyes, even in the way the blood clots we should find this profound and pervading difference from ourselves. A lobster is a_ living creature ; his individual cells work in much the same way as ours do and their chemical and physical needs are the same as ours. But all the arrangements which are made to satisfy those needs are different. He is a cell-community, as we are, but he is a community organized ab initio on a different plan.

§ 2 The Arthropod Plan of Structure

A number of different kinds of animal are classified with the lobster in a separate phylum, the Arthropoda, just as the vertebrates are classified together in one phylum, because in the general plan of their organization they resemble each other and are profoundly different from other living creatures. We have introduced ourselves to the group by examining in some detail one of its members, and before passing to a systematic survey of the others it will be best to make clear how far the points we observed are general features of the arthropods and

THE ARTHROPODS

how far they are peculiar to the lobster or to that smaller group, the class Crustacea, to which he belongs.

The abdomen of a lobster, we noted, is composed of a series of hard segments, flexibly jointed together; each segment bears a pair of appendages. The cephalothorax, on the other hand, has a single shield of armour and bears thirteen pairs of limbs. Now, when we examine other arthropods, we find that the segmental plan of structure, as illustrated by the lobster’s abdomen, is a very important one. It underlies the organization of the whole group. In many arthropods, such as the centipedes or some of the smaller and less specialized crustaceans, the greater part of the body consists of a chain of hard segments jointed together and each bearing a pair of appendages and, indeed, all the members of the group are best regarded as modifications of chains of this type. In a lobster the first thirteen segments are solidly welded together, forming the cephalothorax, only the appendages remaining distinct from each other ; in other arthropods the underlying plan has been modified in other ways. Imagine a chain of segments—an enormously extended lobster abdomen ; each segment with an armour skeleton outside it and bearing a pair of jointed appendages. That would be a diagrammatic, generalized arthropod. By playing on such a scheme, by varying the number of segments, by welding some together and keeping others distinct, by modifying and adapting different appendages for the performance of different functions, all the various existing arthropods could be evoked.

The idea that the arthropods are variations upon a pervading segmental chain can be applied also to their internal architecture. All arthropods live inside their skeletons ; all have a general body-cavity filled with pale blue blood ; all have a nervous system running along the lower sides, and if, as is usual, they possess a heart it is in the back. But the various structures may shift to and fro along the length of the body, from segment to segment. The heart, for example, may be a relatively small organ placed in the middle of the back as in the lobster, or it may be a long tube running down most of the length of the body, with a pair of valved openings in each segment. The position of the excretory, respiratory, and reproductive organs varies widely. And the structure of the digestive organs varies—there may, for example, be no gastric mill.

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