GENERAL ORGANOLOGY. 



for water-breathing; in the latter, on the contrary, it is the 

 skin. On the side of the vertebrates the only exceptions 

 are most amphibians and a (c\\ fishes (Protopterus), in which 

 the gills are tuftlike projections of the skin (Figs. 4 and 

 5, p. 42-43); while among the invertebrates, the tunicatcs 

 and Entcropncnsta, where the digestive tract forms the 

 gills, take a special position. 



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 their goal, 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 interpo- 

 lated between these, and the body 

 consequently becomes more bulky, 

 there is found usually some appa- 

 ratus for distributing the food. 



FIG. 60. FIG. 61. 



FIG. 60 Leftoplana tremellaris. a, mouth ; fr, buccal cavity ; c, opening of the head of 

 the pharynx into the buccal cavity ; d, central stomach ; e, branched entodermal gut ; 

 f, ganglion; g, testicle; /;, seminal vesicle; , uterus ; /, receptaculum seminis ; /, 

 female sexual opening 



FIG. 61. Schema of circulation of the blood. , arteries ; c, capillaries ; /*, auricle ; k, ven- 

 tricle ; /, valves ; /, pericardium ; 7', veins. 



The simplest way is when the digestive tract departs from 

 the character of a straight tube by branching, and by 



