THE INVERTEBRATE SKELETON The materials of the skeleton are very variable; it may, as in the case of the Vertebrate notochord, consist of living cells which have acquired rigidity by the accumulation of water under pres sure in vacuoles and their cytoplasm, that is, by turgones. More usually it consists of non-living materials laid down by cells either within their cytoplasm or outside their surface membrane.
In the case of an exoskeleton, the process of skeleton formation begins with the formation on the outer surface of the animal of a thin layer, usually of an organic substance which forms a cuticle, a structure exactly similar theoretically to a cell wall, but forming a continuous sheet over an epithelium, and resulting from the activities of many cells. This cuticle may remain as a thin layer, serving to resist the mechanical wear to which the outer surface of all animals is subjected, added to only sufficiently to replace loss by abrasion. But in many cases the cuticle is so much thick ened that it becomes inflexible, and gives a permanent form to the part of the animal which it surrounds. It may serve, even when thin, to cement a sedentary animal to its base. The cuticle usually consists of a substance which is a protein, though in Tunicates the test, which is, in effect, a much modified cuticle, is of tunicin, a cellulose. The scleroproteins which compose cuticles vary much in their nature, chitin, which is chemically, perhaps, as closely allied to cellulose as to the proteins, is the most widespread and important. The organic basis of the cuticle may serve as a matrix in which mineral salts may be laid down, so as to harden and still further stiffen the exoskeleton. These salts are usually mixtures of phosphates of calcium and magnesium, or calcium carbonate, in the crystalline form of arragonite or calcite. An exoskeleton of this kind is found in certain Protozoa, in Hydo medusae, Gorgonians, and Zoantharia amongst the Coelenterates, in Polyzoa, Brachiopoda, Mollusca, and reaches its highest devel opment amongst the Arthropoda.
In Coelenterates, the exoskeleton forms a mere support to the animal, and is, except in certain Gorgonians, inflexible and con tinuous. In Brachiopoda and Mollusca, it forms a shell to which muscles are attached, so that movements of the animal with respect to the shell and of the different parts of the skeleton can be brought about. The shells of Brachiopods and Molluscs are often beautiful mechanical devices provided with interlock ing teeth which ensure accurate closure, and presenting, in their shape and special structure, a very great strength for their weight.
The exoskeleton of the Arthropods is far more complex, although actually continuous, not only over the whole external surface, but into those anterior and posterior portions of the gut which are lined by ectoderm. It is functionally divided into a series of segments or somites consisting of a ring of hard chitin, often more or less calcified, which are connected together by areas of thinner and flexible chitin, so that they may move on one another. Each
segment may bear a pair of appendages, each of which is covered by a chitinous cuticle divided into segments movable on one another, though, like those of the body, connected by flexible rings.
The individual sclerites, the hard areas of cuticle, may become hinged to one another by special processes, and are, in all cases, moved by muscles which lie inside them. In such cases as the claw of a lobster, special processes arising from the exoskeleton push their way into the body of the animal in order to give favourable points of attachment to muscles.
The existence of this exoskeleton, which cannot, when once made, increase in area, and whose existence is necessary for the functioning of the animal, renders continuous growth impossible. and has led to the strange process of ecdysis. After a certain pe riod the cuticle cracks along definite lines and the animal crawls out of it, leaving behind the complete structure, retaining the whole of the external surface and even part of the lining of the gut. The animal so freed of restraint very rapidly enlarges and then a new cuticle is formed and hardens to form a new skeleton. Details of the structure of these skeletons must be sought in the separate articles.
An endoskeleton exists in Protozoa, sponges, some Alcyonaria, Brachiopoda, Echinodermata, Chordata and Cephalopod molluscs.
The protozoan endoskeleton may be protein in nature, or it may consist of calcium and magnesium carbonates, silica, or even in one case, of strontium sulphate. It is necessarily intracellular, being completely covered by the cytoplasm of the single cell of which the animal consists.
The sponge skeleton appears always to begin as a series of small rods of some organic material on which spicules of calcium car bonate or of silica are formed. These spicules, in some cases, at any rate, are formed within the bodies of cells, or of fused groups of cells. They arise as isolated structures, and may retain their independence throughout the life of the animal, or they may fuse to form more elaborate structures. Though usually small, they may become gigantic, several feet in length in the case of the anchoring spicules of the glass rope sponge and its allies.
In addition to the spicules, many sponges possess a skeleton of a protein substance, spongin, which extends throughout their sub stance but is usually regarded as an exoskeleton. The endoskeleton of the Alcyonaria consists of spicules which are of intracellular formation that may remain isolated or may be fused into such a substance as precious coral. The Echinoderm skeleton is of meso dermal origin, and is unique in that each of the elements of which it is composed consists of a single crystal of calcite. It is very highly developed in most forms, the individual elements articula ting with one another, often by elaborate joints, and being mov able by a highly-developed musculature. (D. M. S. W.)