216 University of California Publications in Zoology [Vol. 11 



V. GENERAL RESPONSES TO STIMULI 

 1. Formation of Collections 



Often in the aquaria the nephelid leeches may be seen in 

 twisted masses, their posterior ends attached, but usually the 

 anterior suckers free. This is particularly noticeable when no 

 large solid object, such as a stone, is present for them to crawl 

 under and satisfy their stereotropic propensities. 



Smith (1902), worliing on AUolobophora foetida, finds: "If 

 a number of worms are put down near each other they soon 

 crawl together and coil into a heap. ... It is a common obser- 

 vation that earthworms have the habit of lying coiled together 

 in masses. Darwin refers to this habit of Lumhricus in his 

 'Formation of Vegetable Mould,' and Hofmeister (1845) says 

 earthworms pass the winter, either singly or rolled up with 

 others, at the bottom of their burrows." Pearl (1903) has 

 recorded the formation of collections in planarians in the angles 

 of the dish in which they may be kept in the laboratory. Riley 

 (1912) observed in the dragon-fly njTnphs a very pronounced 

 tendency towards the formation of groups, and a similar con- 

 dition has been described by the Severins (1911) for some of 

 the aquatic Hemiptera. 



In order to determine what effect a lowering of temperature 

 might have on this formation of collections, a dozen leeches were 

 placed in a crj'stallization dish in a tightly closed icebox at a 

 temperature of about 10° C. A control was operated on top of 

 this, lighted by a practically uniform illumination from a sky- 

 light above. This was continued for a couple of days, and a 

 more manifest tendency towards the formation of collections 

 was observed in the control than in the dish in the refrigerator. 

 The chief influence of the cold seemed to be that of lowering 

 the vitality of the animal and causing it to remain quiet for the 

 greater part of the time. Thus temperature differences uniform 

 throughout the dish seem a negligible factor in producing these 

 collections, which is perhaps to be explained by the fact that 

 the bod.v of so lowly organized an animal as the leech very quickly 

 takes on the temperature of the surrounding medium. 



