138 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



These skeletal units are strikingly different, white limy ossicles in starfishes, 

 glassy spicules in fantastic shapes and netted fibers in sponges. 



Temporary Skeletons. Such skeletons are shed and replaced throughout the 

 growing period of the animal. The peak achievements in invertebrate skeletons 

 are the jointed ones of insects and other arthropods that are shed and replaced 

 by larger ones as their owners grow (Fig. 9.2). A new shell is formed before 

 the old one is shed and while the new cover is still soft and pliable it stretches 

 enough to allow for another interval of growth (Fig. 9.3). Most insect skele- 

 tons are delicately wrought; those of moths and butterflies are covered with 

 scales many of these lined with extraordinarily fine grooves. At the other 

 extreme is that of the male Hercules beetle of tropical America, nearly five 

 inches long, with heavy headgear that occupies a third the length of its body. 

 Aquatic species are larger than the related land forms; crabs and lobsters have 

 the heaviest skeletons of the arthropods. Yet when lobsters are submerged in 



Fig. 9.3. Dorsal shells (carapace) of the same crab before and after molting. 

 A, hard shell that was recently shed; B, larger new shell that stretched and is still 

 soft. Crab, Loxorhynchus grandis, Pacific Coast. (Courtesy, MacGinitie and Mac- 

 Ginitie: Natural History of Marine Animals. New York, McGraw-Hill Book Co., 

 Inc., 1949.) 



their native sea water they are so buoyed up by it that the tips of their claws 

 touch the rocks as lightly as if they were engaged in a ballet. 



Joints. Joints are the places where adjacent parts of a skeleton join, often 

 closely fitted together. In lobsters and other arthropods the outer covering or 

 exoskeleton is continuous over them, yet it is so thin and pliable that the 

 joint bends easily. Joints are highly developed in the skeletons of insects and 

 vertebrates, two dominant groups of animals. Those of invertebrates began 

 as creases in the epidermis and cuticle such as are so clearly visible in earth- 

 worms. As an insect breathes, its abdomen rhythmically lengthens and shortens 

 at the telescopic joints. When air enters the body, the plates of the skeleton 

 move apart, stretching the soft membrane between them (Fig. 9.4). Alter- 



