S6 PLANARIA NIGRA. 



wards a point ; whereas it ordinarily forms 

 a regular elliptical outline, unless when pro- 

 duced to an acute angle by extreme exten- 

 sion. Possibly, however, the difference may 

 centre in the progress of reproduction, 

 whereof the commencement has not been 

 seen. Very minute dark or light grey pla- 

 nariae sometimes dwell along with the rest, 

 of which the tail is more acute in propor- 

 tion than that of the larger ones. Three 

 of these, a line and a half long, being sub- 

 jected to the microscope, traversed the 

 glass slider with great rapidity, much ex- 

 ceeding the usual progression of the black 

 planaria. A scanty row of very dark specks 

 environed the anterior margin ; and the in- 

 ternal organisation was sensibly exposed 

 through the gelatinous texture of the body. 

 But a series of experiments would be requi- 

 red to entitle us to deny their identity with 

 the young of the common species ; first, be- 

 cause the accelerated motion might be ex- 

 cited, from the susceptibility of a small quan- 

 tity of water, of an increased atmospherical 



