52 Art. 2.— T. Kaburaki : 



bits of weed, probably as mucli for the supply of oxygen and con- 

 sequent purer water as for a hiding place. In balanced aquarium, 

 they nre therefore capable of living in the same manner as in 

 nature. 



Reactions. — Ail the planarians are, to some extent, sensitive 

 to light, being definitely and conspicuously negative in their 

 relation both to difïused and localized light. Consequently they 

 ordinarily occupy a position beneath stones or other sunken 

 materials, and move about more l)y night than by day. If placed 

 in a dish, tiiey investigate very tlioroughly every part and in- 

 variably come to rest in the situation of greatest shadow in the day 

 time. Their ability to estimate the size and density of the shadow 

 is evidently keen, since they seem able to distinguish even small 

 gradations. The eyes are an aid in distinguishing différences in 

 light intensity as well as the direction from which light comes, but 

 the presence or absence of these organs cannot be taken as the 

 main factor in this respect, for species which are wholly destitute 

 of them may possess the ability to distinguish between light and 

 darkness. Another fact that is perhaps to be correlated with light 

 is the relative amount of colour. The pigments in the epidermis 

 or subcutaneous tissue were regarded as arising from the direct in- 

 fluence of light, and, as a necessaiy corollary to this view, it was 

 also asserted that the absence of light always prevented the forma- 

 tion of pigments, or destroyed that which was already formed. The 

 fact that Bd. annandalel, occurring in the deepest parts of Lake 

 Biwa, is wholly devoid of any ti'ace of pigments appears to afford 

 a striking evidence of the accuracy of this statement. 



Locomotor 3Iovement. — As already briefly stated in my paper 

 deaUng with the marine Triclads, there can usuall}^ be distinguished 

 three sorts of locomotor movements, gliding, crawling and so-called 

 swimming, Gliding is the most usual mode of progressive move- 

 ment when the animal is not particular!}^ stirred up, while crawling 

 is seen when it is strongly stimulated in certain ways. When the 

 worm starts from a resting condition the nature of the movement, 

 i.e. whether gliding or crawling, depends in a large measure on the 

 intensity of the stimulus which starts it. If the resting animal is 



