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 are numbered identically. 



