MOVEMENTS, ETC., OF PHESH-WATKB PLANAEIANS. 527 



individuals being found in considerably greater numbers in 

 the spring than at any other time in the year so far as my 

 observations go. This seems to be true also of the rhab- 

 docoeles. 



Relatively the most abundant species of planarians in this 

 region aie Planaria dorotocephala and maculata. The 

 numbers of these two are about equal, with a slight advantage 

 in favour of P. dorotocephala. Next in abundance I have 

 found to be P. gonocephalaj but this is considerably below 

 the other two. Finally comes Dendrocoelum, sp., which I 

 have never found in an abundance to be compared with the 

 species of Planaria. 



II. Activities. 



The movements of planarians will be discussed in detail in 

 a later section of the paper,^ but it is desired to take up here 

 certain general activities and relations to the environment 

 which properly fall under the general subject of natural 

 history. 



The first of these subjects is — 



a. Sensitivity. — The flat-worm is extremely sensitive to 

 a variety of stimuli. Among the different stimuli which pro- 

 duce specific reactions, and to which we must therefore 

 conclude it is sensitive, are the following : — Mechanical 

 disturbances of the general environment (shaking, jarring, 

 movement of Avater, etc.), contact (localised mechanical 

 stimulation), chemical changes in the environment (in the 

 widest sense, including food substances), light, the electric 

 current, desiccation, a current of water, and heat. 



Its extreme sensitivity, which makes it responsive to very 

 slight changes in the environment, mny bo shown by a very 



' It may be slated lieie, for the conveuience of llie reader before reacliinjf 

 the full discussion of the movements, that the progressive movements of 

 triclads are of two sorts. These are (a) gliding movements, in which there is 

 little or no muscular action ; and (6) crawling movement?, in which the motion 

 is effected by muscular contractions involving the wiiole body. Tlie crawling 

 has some general resemblances to the method of progression observed in a 

 leech of the genus Clepsine. 



