112 PLANARIA ARETHUSA. 



in the history of the genus, no less remark- 

 able than those distinguishing the indivi- 

 dual species that compose it. Still, it is a 

 conclusion which we should beware of adopt- 

 ing,, without the test of rigorous experi- 

 ment. 



The powerful reproductive principle re- 

 sident in the various parts of the arethusa 

 is seen in its multiplication, from artificial 

 sections. All are speedily regenerated in- 

 to entire animals, their progress and per- 

 fection being dependent on the usual laws 

 which regulate the evolution of defective 

 parts. Monstrosities are sometimes found, 

 of which an example has been already quo- 

 ted ; and at another time, one was taken 

 with two complete tails of considerable 

 length, neither of them resembling an ac- 

 cidental production ; so that it was doubt- 

 ful which was original, or if either had pre- 

 ceded its fellow. Bifurcation of the tail is 

 the most common kind of monstrosity in 

 the genus, but it is probably to be in gene- 

 ral ascribed to accident ; and we can more 



