GENERAL ZOOLOGY 



Fig. 14.16. A branchiobdellid worm, Camhanm'o/a philadelphico. 

 This small olia;ochaete, with superficial resemblances to a leech, 

 clins;s to its host, the crayfish, by means of the expanded posterior 

 sucker. (Photograph courtesy General Biological Supply House, 

 Inc.) 



lining the digestive cavity. The submucosa and the muscular layer contain 

 numerous capillaries important in the absorption of products of digestion. 

 The digestive epithelium is composed of columnar cells and gland cells. As in 

 other metazoans, this is the layer from which digestive enzymes are secreted 

 into the lumen and through which absorption occurs. The extensive, dorsal 

 typhlosole of the stomach-intestine is analogous to the numerous villi and 

 rugae of the vertebrate intestine; all function to increase the secretory and 

 absorptive area of the wall of the gut. 



Other Oligochaetes. There are well over 1800 species of earthworms 

 and hundreds of additional species of aquatic oligochaetes. With this in 

 mind, and considering also the diverse conditions of existence to which these 

 various forms have become adapted, we should not expect all oligochaetes to 

 resemble Lumbricus in detail. Nevertheless, Lumbncus is fairly typical of the 

 general annelid grade of organization. Other oligochaetes, ranging in size 

 from tropical earthworms which may reach 6 or 8 feet in length to fresh- 

 water worms measuring less than a millimeter, present many interesting 

 structural and functional modifications. Tubifex (Fig. 14.15), for example, a 

 small aquatic form, customarily lives with its anterior end buried in mud. 

 The posterior portion is extended and waves about in the water, seeking a 

 level containing adequate amounts of dissolved oxygen. In Tubifex gas 

 exchange occurs chiefly through the wall of the. intestine, cutaneous gas ex- 

 change being subsidiary. In these and many other small oligochaetes, the 

 capillary beds characteristically present in the intestinal submucosa of 

 Lumbricus are represented by large, continuous blood sinuses, although the 



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