92 EXPERIMENTS UPON PLANARIA 



to preserve them in a healthy condition in their confined state, 

 and therefore did not actually watch the daily growth of the new 

 arms. 



In an experiment upon a more highly organized animal, I was 

 better favored, and succeeded, after cutting it in two across the 

 middle of the body, in obtaining a most ample proof of its 

 regenerative powers. It belongs to the class of Worms, and is 

 known as the Flat-worm, or Planaria, (fig. 47). 

 There are numerous species in our ponds, where 

 they creep over the surfaces of stones and 

 J6 aquatic plants. The one which is figured here 

 is very common, and readily detected and rec- 

 I ognized by its opaque white color, and the liver- 

 colored ramifications of its intestine. The 

 mouth is at the middle of the body, on the 

 Fig. 47. under side, and opens into a short, thick, cylin- 



drical proboscis, (p,} which when retracted lies in an oval cavity. 

 From the latter, the intestine extends in two, or rather in three 

 directions ; anteriorly (c) it reaches, in a median course, nearly 

 to the end of the head, and gives off numerous, irregular, lateral 

 branches toward the sides of the body ; in the opposite direction 

 it proceeds in two nearly parallel lines (e) backwards, on each 

 side of the proboscis, until it thins out into very minute branch- 

 lets in the tail. Each limb gives off numerous ramifications 

 toward the edge of the body, on that side in which it lies. The 

 reproductive organs are situated mostly in the posterior half of 

 the body, and their opening (d) lies half way between the pro- 

 boscis and the end of the tail. The nervous system consists 

 principally of an oval mass, which lies across the anterior end of 

 the body, and two slender, thread-like, indistinct nerves, which 

 extend backward along the lower side of the intestine. If, 

 now, the animal is cut in two, at a point (through a, b) just 



Fig. 47. Dendrocoslum lacteum. (Est. ? 3 diam. A milk-white, fresh- 

 water Planaria. p, the proboscis ; c, the anterior branch of the intestine ; e, the 

 posterior branches of the intestine ; a, b, point of junction of c and e, d, the 

 aperture of the reproductive organs. Original. 



