NUTRITION 



This general plan of the alimentary system, which is 

 common to most of. the coelomaria in its chief features, 

 is very much modified in the various groups of these 

 animals and adapted to their several conditions of nu- 

 trition. The simplest structures are found in many of 

 the vermalia; the lowest forms of these, the rotifers, and 

 especially the gastrotricha, still closely resemble their 

 platode ancestors, the turbellaria. The higher type of 

 animal-stems which have been evolved from them are 

 partly distinguished by special structures. Thus the 

 moUusks have a characteristic masticating apparatus; 

 on their tongue there is a hard plate (radula) armed 

 with a number of teeth, which grinds against a hard up- 

 per jaw, and so breaks up the food. In most of the ar- 

 ticulates this work is done by side-jaws, which consist 

 of hard rods and represent modified bones. The verte- 

 brates and the closely related tunicates are distinguished 

 by the conversion of the first sections of the alimentary 

 canal into a characteristic respiratory apparatus (gills). 

 But the construction of the various sections of the gastro- 

 canal also varies a good deal in the small groups of the 

 coelomaria, as it depends to a great extent on the nature 

 of the food and the conditions in which it is got and 

 prepared. The largest expenditure of mechanical and 

 chemical energy is needed for a voluminous solid vege- 

 tal diet. Hence the alimentary canal and its many 

 appendages are longest and most complicated in the 

 plant-eating snails, leaf-eating insects, and grass-eating 

 ruminants. On the other hand, thfey are shortest and 

 simplest in parasitic coelomaria, which derive their fluid 

 food already prepared from the contents of another ani- 

 mal's intestines. In these cases the gut may altogether 

 atrophy; as in the acanthocephala among the vermalia, 

 the entoconcha among the moUusks, and the sacculina 

 among the Crustacea. 



The greater the extent of the body, and the more 



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