. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 78 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. coloured and variously shaped shells of these animals. Integumen- tary glands and aggregations of glands may also acquire a relation to the acquisition of food (spinning gland* of Spiders). Finally, mucous glands are very widely present in the skin of animals which live in damp localities (Amphibia, Snails) and in water (Fishes, Annelids. Medusae). ORGANS OF ANIMAL LIFE. Of the so-called animal functions, that of locomotion is the most
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 78 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. coloured and variously shaped shells of these animals. Integumen- tary glands and aggregations of glands may also acquire a relation to the acquisition of food (spinning gland* of Spiders). Finally, mucous glands are very widely present in the skin of animals which live in damp localities (Amphibia, Snails) and in water (Fishes, Annelids. Medusae). ORGANS OF ANIMAL LIFE. Of the so-called animal functions, that of locomotion is the most conspicuous. Animals perform movements for the purpose of procuring food and escaping from their enemies. The muscles used f or locomotion are, as a rule, and especially in the simpler forms, intimately united with the skin, and give rise to a muscular body wall (Worms), the alternate shorten- ing and elongation of which brings about a movement of the body. The muscles mav also be especially concentrated in parts of the body wall, , in the subuni- brellar surface of Medusze beneath the supporting gelatinous tissue, or in the ventral surface of the body giving rise to a foot-like organ (Molluscs), or they may be broken up into a series of successive and similar segments (Annelids, Arthropods, Vertebrates). The latter arrangement prepares the way for the rapid and more complete form of movement found in animals in which the hard parts also, whether exoskeletal (Arthropods) or endoskeletal (Vertebrata), have become divided into a series of longitudinally arranged segments or rings, which offer a firm attach- ment to and are moved by the segments of the muscular system. By this arrangement more powerful muscular actions are rendered possible. Thus it becomes indispensable that hard parts should be developed to act as a skeletal support for the soft parts, and also to protect them. The skeletal structures may be external, in which case they have the form either of external shell
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