Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 137 



or in the air. These skeletons are calcareous (limy), silicious (glassy), and 

 chitinous (horny), or are combinations of these. Those of aquatic animals 

 often have flotation devices, cavities that contain air or gas, fat, and oil 

 droplets. In the larger groups of multicellular invertebrates there are three 

 general types of skeletons. 



Permanent Skeletons. Clams, snails, and other mollusks have but one skele- 

 ton throughout life enlarging it as their bodies grow. Although the molluscan 

 shell is not called a skeleton it has the requirements of one. In clams the oldest 

 part of the shell is the hinge region from which larger and larger concentric 

 ridges show where new secretion has been added (Fig. 9.2). The swiftest 

 mollusks are the squids whose skeletons are completely hidden by a fleshy 

 mantle. 



An exoskeleton may be a network of minute units, or a mosaic of closely 

 fitted plates. As the animal grows, the units are enlarged or new ones added. 



Clam 



Starfish , ossicles (black) 



Sponge, spicules 



Lobster 



SKELETONS OF INVERTEBRATES 



Fig. 9.2. Skeletons of invertebrates. Permanent: clam with lines showing the 

 additions to the shell throughout life; cut across the arm of a starfish showing the 

 limy ossicles (shaded) embedded in the flesh of the body wall. Left lower: spicules 

 of fresh-water sponge that form a net-like support in the body wall. Temporary: 

 lobster whose skeleton is periodically replaced by a new one as long as the animal's 

 growth continues. 



