28 



GENERAL PRINCIPLES OF ZOOLOGY 



shown that they pass through a nauplius-stage (fig. 6a), characteristic 

 of most Crustacea, and that they then assume the shape of small Crustacea 

 (fig. 6, b), like Cyclops (fig. 7, A), so widely distributed in fresh waters. 

 Very often the males make a halt in the cyclops-stage while the female 



FIG. 5. Siredon pisciformis (larva of Amblystoma tigrinum) (after Dumeril and 



Bibron). 



develops farther and assumes a shapeless form, so that there arises a very 

 remarkable sexual dimorphism (fig. 8). All these examples, which can 

 be multiplied by hundreds, can be explained in the same way. The higher 



FIG. 6. 



\ I 



Ichtheres pcrcarum. a, nauplius-, /;, cyclops-stage; c, adult female (after 



Claus). 



forms pass through the stage of the lower, because they spring from an- 

 cestors which were more or less similar to the latter. Man in his em- 

 bryological development passes through the fish stage, the frog the 

 perennibranchiate stage, the parasitic crustacean first the nauplius- 



