AGE DIFFERENCES IN SUSCEPTIBILITY 



93 



The general plan of internal structure of 

 other related species is much the same, but 

 they differ in shape and general appearance. 

 Planaria macidata (Fig. 7) does not attain as 

 large a size as P. dorotocephala and is less 

 active. The head differs in shape from that 

 of P. dorotocephala and the pigment is dis- 

 tributed in large spots. P. velata (Fig. 8) is 

 more slender, somewhat less flattened, and 

 without the pointed cephalic lobes. The 

 younger worms are almost black, but become 

 light gray with advancing age. 



Various other flatworms, protozoa, the 

 fresh-water hydra, and several marine 

 hydroids have been used in comparative ex- 

 periments. 



AGE DIFFERENCES IN SUSCEPTIBILITY IN 



Planaria maculata 



Animals of this species kept in the labora- 

 tory and fed become sexually mature and 

 deposit egg capsules containing fertilized 

 eggs, and from these capsules the young 

 worms emerge in about four weeks at 

 ordinary temperatures. When first hatched 

 the young worms possess the form of the 

 adult, but are only about two millimeters in 

 length, while in my stock the old, sexually 

 mature worms, which were laying eggs, were 

 about twelve millimeters long. 



Fig. 9 shows the susceptibility curves (see 

 pp. 80-82) of young and old animals of 

 this species to potassium cyanide, o.ooi mol. 

 The curve ah gives the susceptibility for ten 

 newly hatched worms, the curve cd, that 

 for ten full-grown sexually mature worms 

 about twelve millimeters in length. The 



j^fl'' 



-p/i 



VI 



Fig. 6. — Planaria 

 dorotocephala: w, mouth; 

 ph, phar>Tix; al, alimcn- 

 tar>- tract; us, nervous 

 system. 



