GENERAL OROANOLOGT. 109 



may become an accessory resjiiratory organ, being filled from time 

 to time with fresli water. 



Aerial Respiration. — In the air-breathing animals the respira- 

 tory apjDaratns is derived either from the digestive canal or from 

 the skin. With the vertebrates the former is the case, since the 

 lungs, either directly or by the mediation of the trachea and bronchi, 

 are in connexion with the lumen of the digestive tract. On the 

 contrary, in the case of invertebrate animals (snails and spiders) 

 when the term ' lung ' is used, it refers always to an invagination 

 or sac of the skin; of such a nature are the tracheas of insects, 

 tubes containing air, beginning at the surface of the body with a 

 hole or stigma, and branching internally (fig. .59, st). 



Distinctions between the Respiratory Systems of Chordates 

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

 between the respiratory systems of vertebrate and invertebrate 

 animals : in the former, the digestive tract, or derivatives from it, 

 are respiratory; in the latter, on the contrary, it is the skin. On 

 the side of the vertebrates the only exceptions are most amphibians 

 and a few fishes {Protopterus), in which the gills are tuftlike pro- 

 jections of the skin (figs. 4 and 5) ; while among the invertebrates 

 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 tissues, there is need of no special organs, so long 

 as the body consists of only two thin epithelial layers, the ectoderm 

 and entoderm. When, however, a third, a mesodermal, layer is 

 interpolated between these, and the body consequently becomes 

 more bulky, there is usually some apparatus for distributing the 

 food. The simplest is when the digestive tract departs from the 

 character of a straight tube and branches, and by means of these 

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

 then of a fjastro-vascidar systein, because the alimentary canal 

 itself takes on the function and the branching arrangement 

 generally characteristic of the vessels or ' vascula ' (fig. G3). 



Coelom. — The ccelom or enteroca3le is apjjarently derived from 

 a jiair of gastric diverticula which have become completely cut off 

 from the archenteron (comp)are development of mesoderm, iiifru). 

 It is a cavity pushed in between the intestinal tract and the body- 

 wall, is lined by a special epithelium, the peritoneum, and encloses 



