3G 



OENERAL PItlNCIPLES OF ZOOLOGY. 



resemblances to tlie lowest vertebrates, tlie fishes. Like these it 

 has gill-slits, the same arrangement of the heart and of the arterial 

 vessels, certain fundamental features in the development of the 

 skeleton, etc. (2) Frogs in their tadpole stage have an organiza- 

 tion similar to that which remains permanent in the case of 

 certain Amphibia, the Perennibranchiata (fig. 5), which stand 





ViG. r^—Sircdnn 2jisci/o»-)ni« (larvii of Arnhh/xtoma liuriniim). (After Dum^ril and 



Bibroii.) 



lower in the system; they have a swimming tail and tuft-like gills, 

 which are lacking in the adult frog. (3) There are certain para- 

 sitic Crustacea, which live upon the gills of fishes, and seem not 



Fig. f>.—AcWiercs pcmini 



ir, Tiaiipliiis 

 (.\fter C'laus.) 



adult female. 



at all like their relatives. They are sliapeless masses wliicli were 

 formerly regarded as para,sitic^ worms. Their systematic position 

 was only determined l)y their embryology (fig. 0). Here it is 



