HISTORY OF ZOOLOGY. 



43 



blances 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 organization similar to that, which 

 remains permanent in the case of certain amphibia, the 

 Perennibranchiata (Fig. 5), which stand lower in the sys- 



FIG. 5. Siredon piscifonnis (Axolotl). (After DumeVil and Bibron.) 



tern; they have a rudimentary tail and tuft-like gills, 

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

 tain parasitic crabs, which live upon the gills of fishes, and 



FIG. 6. Achtheres Percarutn. ,i, nauplius-, t>, cyclops-stage ; c, female. (After Claus.) 



seem not at all like the other crabs. They are shapeless 

 masses which were formerly regarded as parasitic worms. 

 Their systematic position can only be determined through 



