36 



GENERAL PRINCIPLES OF ZOOLOGY. 



resemblances to the lowest vertebrates, the 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 



FIG. 5. Siredon pisciformis (larva of Amblystoma tigrinum). (After Dum6ril and 



Bibron.) 



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. 6. Achtheres percarum. a, nauplius-, h, cyclops-stage ; c, adult female. 

 (After Claus.) 



at all like their relatives. They are shapeless masses which were 

 formerly regarded as parasitic worms. Their systematic position 

 was only determined by their embryology (fig. 6). Here it is 



