BLOOD SYSTEM. 



397 



be classed as a caducibranch j but it still presses on. 

 Finally, it loses its tail and reaches the highest order of 

 amphibia — the Anura. Of course the caducibranchs 

 and perennibranchs pass through similar stages, but 

 stop earlier. So much for the similarity of the onto- 

 genic and taxonomic series. 



Now, there can be no doubt that the phylogenic series 

 is also similar, that the amphibians were evolved from 

 the dipnoan fishes, and that in the course of geologic 

 times they passed through the same stages — i. e., peren- 

 nibranch, caducibranch, and became anurous only in 

 Tertiary times. Thus the three series are similar. All 

 these facts are expressed in the following diagram: 





Anura — *- = 

 Caducibranch-; 

 Perennibranch 

 Fish 



Ontogenic 

 A. 



■iFish^> Perennibranch-* Caducibranch -*Anura 



h — > Perennibranch 



-Fisti- 



-Fish 



Fig. 275. 



SECTION IV. 



Morphology of Respiratory and Circulatory System in 

 Invertebrates. 



Some General Introductory Remarks. — (1) In 



the case of vertebrates we gave first the morphology of 

 the respiratory organs and then of the circulatory organs 

 separately, but in the case of the invertebrates we shall 

 take these together in each class treated. 



(2) In the vertebrates the blood system is a system 

 of closed pipes, continuous and without either discharge 

 or intake, except by exosmose and endosmose. In nearly 



