. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 142 ELEMENTS OF BIOLOGY Protective and Supporting Structures. Many and diverse types of protective and supporting structures occur among animals. Among the sponges supporting structures that consist of rods and spicules are developed in the ectoderm (Fig. 87). In some sponges these are composed of carbonate, in others of silicate; in the com- mon bath sponge a peculiar substance called spongin is formed as the supporting structure. In echinoderms integumentary plates containing calcium car- b
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 142 ELEMENTS OF BIOLOGY Protective and Supporting Structures. Many and diverse types of protective and supporting structures occur among animals. Among the sponges supporting structures that consist of rods and spicules are developed in the ectoderm (Fig. 87). In some sponges these are composed of carbonate, in others of silicate; in the com- mon bath sponge a peculiar substance called spongin is formed as the supporting structure. In echinoderms integumentary plates containing calcium car- bonate serve for protection and support (Fig. 88). In the mol- luscs the shell is essentially cal- cium carbonate, secreted by the cells of the outer layers of the body wall (Fig. 89). The ar- thropod body is enclosed in a series of tough movable plates composed of an organic sub- stance called CHITIN (Fig. 90). These outer plates serve as ful- cra for the attachment of mus- cles, so that in the insect for instance, the leg has its skeleton external and the muscles inter- nal, the reverse of the relation between skeleton and muscles in the vertebrates. Surface and integumentary protective and supporting structures form what is known as the exoskeleton. The Chordate Exoskeleton. Among the chordates, particu- larly in the vertebrates, the exoskeleton is frequently extremely well developed. The vertebrate exoskeleton consists primarily of scales. In its most primitive type, in the elasmobranch fishes of which the shark may serve as an example, the scale consists of two portions, an enameled spine formed by the metabolism of ectodermal cells, and. Fig. 86.—An illustration of serial ho- mology. A is a. mouth part (second maxilliped) and B the second walking leg of a lobster. Although quite different in appearance and employed by the animal for quite different uses, both represent modifications of a basic type of appendage. In the figures the homologous parts dis- tinctly visible a
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