394 L - H - HYMAN. 



compared. This age difference in susceptibility to toxic solu- 

 tions has been observed in Paramecium, three species of Hydra, 

 a number of colonial hydroids and hydromedusae, several species 

 of planarians, and several small aquatic oligochaetes; and the 

 rise in metabolic rate which is a feature of early development has 

 also been demonstrated by this method. 



Not only does the rate of respiratory exchange in organisms 

 vary inversely with age but many physiological activities exhibit 

 the same relation. Bert ('70) was among the first to observe 

 this fact. He noted that the rate of respiratory movements is 

 faster in small than in large individuals of the same and related 

 species. Ducceschi ('03) studied the rate of movement of the 

 fins, tail, fin membrane, and operculum of a large number of fish 

 of different sizes; the rate of movement of the maxillipeds, 

 abdominal appendages, and claws of several Crustacea; and of the 

 fins and mantle of some cephalopods; and found that it varies 

 inversely as the size of the animal. Mayer ('06) and Child 

 ('18) observed that the rate of contraction of the bells of medusae 

 is faster the smaller the animal. The number of respiratory 

 movements per minute of Octopus is greater the smaller the 

 individual (Polimanti, '13). In four species of sea-cucumber, 

 Crozier ('16) observed that the pulsation of the cloaca is more 

 rapid the smaller the animal. Hecht ('16, '18) noted that 

 small Ascidia pass relatively more water through their siphons 

 than large ones; and further that the rate of the heart beat in 

 Ascidia varies inversely as the size of the animal. In man, of 

 course, it is well known that the rate of the heart beat is fastest 

 in the foetus and decreases progressively with age. The rate of 

 growth follows a similar law. 



The results of the present experiments are in full accord with 

 those of previous investigators. Small (young) planarians 

 whether asexually or sexually produced, consume more oxygen 

 per unit weight per unit time than large (old) ones. 



II. EXPERIMENTS WITH Planaria dorotocephala. 



Planaria dorotocephala lives in spring-fed marshes in morainic 

 regions near Chicago. It has never been found sexually mature 

 in nature (although sexual maturity has been experimentally 



