GENERAL ZOOLOGY 



external parts, such as the feathery gills and the hair-like sensory setae on the 

 appendages, are covered by a thin layer of the skeleton. Like the cuticle of 

 an annelid, the crustacean exoskeleton is a non-cellular, non-living secretion 

 product of the cells of the epidermis. Basically, the exoskeleton is composed 

 of varying proportions of tough scleroproteins and a characteristic nitrogen- 

 containing polysaccharide called chitin. At points of flexure, as between 

 somites of the abdomen and between divisions of the appendages, the skeleton 

 is thin and flexible. In its thicker portions, as in the carapace, the organic 

 substance of the basic skeleton is hardened by the addition of calcium 

 carbonate. The exoskeleton of the crayfish may be progressively thickened 

 by the deposition of additional material, but it cannot stretch laterally except 

 in the early stages of its formation, before the calcium carbonate has been 

 deposited. As a result of this mechanical relationship between the skeleton 

 and the completely enclosed body, growth cannot occur by continual ad- 

 ditions to the skeleton, as it can with the exoskeleton of a mollusk. Hence, 

 the crayfish, like other arthropods, periodically resorbs some of the material 

 of its skeleton and secretes a thin, new skeleton beneath the old one. The old 

 one, including the lining of both ends of the digestive tract, is then shed, or 

 molted. In the few hours immediately following such a molt, the crayfish, 

 covered only by the thin, elastic, flexible new skeleton, imbibes water and 

 swells to a larger size. As the new skeleton progressively hardens during the 

 days that follow, the animal assumes again its normal hard-shelled condition. 

 During its soft-shelled state the crayfish is defenseless; and, since its muscles 

 are attached only to the flexible new skeleton, its powers of movement are 

 much impaired. It usuallv remains in its burrow or otherwise concealed 

 until the skeleton has attained some degree of hardness. The soft-shelled 

 crabs, considered a table delicacy, are merely crabs captured so soon after 

 molting that their shells have not yet hardened. 



Structures and Functions Related to Metabolism. The digestive system 

 consists of the digestive tract and its appended glandular organs (Fig. 15.5). 

 A short esophagus leads from mouth to stomach, the anterior portion of which 

 contains the gastric mill. This structure has teeth or ossicles formed by 

 thickenings of its skeletal lining; the teeth are so arranged that they grind 

 against each other and so complete the mastication of ingested food. Large, 

 paired digestive glands open into the anterior region of the intestine, immedi- 

 ately behind the stomach. Finely divided particles of food are passed through 

 filters in the stomach and into tubular cavities within the digestive glands. 

 These tubules are lined by an epithelium composed of several types of cells: 

 one type secretes digestive enzymes which accelerate the breakdown of food 

 in the tubules; another type functions in ab.sorption of the products of diges- 

 tion and in storage of energy reserves such as glycogen and fat. Undigested 

 residues of food are returned into the cavity of the intestine and carried back 

 through the long hind-gut for egestion at the anus. The intestine apparently 

 serves little or not at all in digestion and absorption; only the portion of the 

 intestine into which the digestive glands open is lined by a mucosa derived 



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