GENERAL ORGANOLOGY 99 



Distinctions between the Respiratory Systems of Chordates 

 and Invertebrates. In general, then, a distinction can be drawn be- 

 tween the respiratory systems of vertebrates and invertebrates: in the 

 former, the digestive tract, or derivatives from it, are respiratory; in the 

 latter, on the contrary, it is the skin. Of the vertebrates the only excep- 

 tions are most amphibians and a few fishes (Protopterus], in which the 

 gills are tuftlike projections of the skin (figs. 4 and 5) ; while among the in- 

 vertebrates some aquatic insects respire by the hinder end of the digestive 

 tract. 



III. Circulatory Apparatus. 



In order that the oxygen, taken up by the respiratory organs, and the 

 constituents of the food digested in the alimentary canal may reach the 

 tissus, there is no need of special organs, so long as the body consists of 

 only two thin epithelial layers, the ectoderm and entoderm. \Yhen, 

 however, a third, a mesodermal, layer is interpolated between these, and 

 the body consequently becomes more bulky, there is usually some appa- 

 ratus for distributing the food. The simplest is when the digestive tract 

 departs from the character of a straight tube and either gives off a few 

 broad sacs (gastral pouches') or it branches, and by means of these 

 branches extends into the various parts of the body. We speak then of a 

 gastro-vascular system, because the alimentary canal itself takes on the 

 function and the branching arrangement generally characteristic of the 

 vessels or 'vascula' (fig. 64). 



Coelorn. The ccelom or enteroccele is apparently derived from a pair 

 of gastric diverticula which have become completely cut off from the 

 archenteron (compare development of mesoderm, infra). It is a right and 

 left cavity pushed in between the intestinal tract and the body- wall, is 

 lined by a special epithelium, the peritoneum, and encloses most of the 

 vegetative organs. If the two halves of the coelom approach, without unit- 

 ing, dorsal and ventral to the gut, the result is dorsal and ventral mem- 

 branes, the mesenteries, which support the alimentary canal (fig. 241). In 

 many invertebrates the ccelom plays an important role in nutrition since 

 it contains a lymphoid fluid, rich in proteids and containing cellular 

 corpuscles; it is also important for excretion since it may communicate 

 with the nephridia (see p. 105) by the ciliated funnels. It loses this 

 significance the more the blood system is developed, and in the vertebrates, 

 so far as nutrition is concerned, it is a rudimentary organ. 



A sharp distinction should be drawn between the coelom and other cavities in 

 the body. Not every 'body cavity' is a crelom, but frequently there occur 

 large spaces which are entirely different in origin and in relations. Frequently, 



