STRUCTURE AND FUNCTION IN INVERTEBRATES 



the esophagus is a passageway; the stomach is the place where digestion and 

 absorption begin; the intestine is the region where these processes are com- 

 pleted; and the terminal region called the rectum forms feces for egestion and 

 may have special ancillary functions such as reabsorption of water. Salivary 

 glands secrete digestive enzymes or merely lubricating fluids. A digestive 

 gland, paired or single, opening into the region between stomach and intes- 

 tine, commonly supplies the principal digestive juices. In certain inverte- 

 brates, as in the crayfish and starfish, such glandular diverticula are also the 

 chief sites of digestion and absorption. Unicellular glands which secrete 

 lubricants or enzymes may also occur throughout the lining of the digestive 

 tract. As in the vertebrate, absorption involves the passage of the products 

 of digestion through the lining of the tract into any blood or lymph spaces 

 in its wall, or directly into the fluid of the body cavity. The parallel with 

 structure and function in the vertebrates is obvious, and it is clear that the 

 same fundamental mechanisms operate in the digestive systems of all animals 

 possessing a complete digestive tract with mouth and anus. 



Circulatory Systems. The circulatory systems of invertebrates are very 

 diverse. In the protozoans cyclotic movements of the cytoplasm often trans- 

 port food vacuoles and absorbed nutrients throughout the cytoplasm, perform- 

 ing a function which corresponds to that of the circulatory systems of 

 metazoans. Another analogous but fundamentally dissimilar mechanism is 

 found in sponges, where currents of water passing through the body furnish 

 a transportation system for food and for excreta. In Coelenterata and 

 Ctenophora, the need for a vascular transport system is obviated by the close 

 relationship existing between the digestive cavity and all other parts of the 

 body. No parts are far removed from the cavity itself, or from its branching 

 extensions in some forms. By muscular contractions, or by flagellary currents 

 in the enteron, food and products of digestion are circulated through the 

 entire body, and the enteron is therefore sometimes called a gastrovascular 

 cavity. Among bilateral forms Platyhelminthes present a similar relationship: 

 there is no anus, and the highly branched enteron, when present, commonly 

 extends throughout the body. A circulatory function is often attributed to 

 the lympth-like fluid which fills the interstices of the mesenchymal mass sur- 

 rounding the organs. In Nemertinea, which have a complete digestive tract, 

 there are blood vessels, although they form a very simple vascular system 

 without a localized propulsive mechanism and without a definitely directed 

 circulation (pp. 340-343). 



The more efficient blood vascular systems of higher invertebrates are char- 

 acteristically provided with some mechanism which propels the blood along a 

 definite circulatory pathway. The simplest of these involves peristaltic con- 

 tractions of the principal blood vessels. This may be illustrated specifically 

 by the circulatory system of an annelid, such as the earthworm (pp. 406- 

 407); here the larger vessels are contractile and so drive the blood through 

 the body in a definite course. The so-called "hearts" of the earthworm are 

 merely enlarged vessels connecting the dorsal and ventral parts of the system. 



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