242 University of California Publications in Zoology [Vol. 11 



In the young of this same species, the deceptive quiet reaction 

 was secured upon contact stimulation just as in the case of the 

 older specimen discussed in a preceding section of this paper. 

 One young specimen was observed to remain perfectly quiet for 

 as long as eight minutes after being lightly stimulated with a 

 fine bristle attached to a wooden handle. Further light stimu- 

 lation of a young leech, when placed in this condition of deceptive 

 quiet, does not produce more than a local contraction, in most 

 cases, just as was found to be the ease with the adult specimens 

 of this same species. 



The young of Glossiphonia were secured in much greater 

 abundance than those of Hemiclepsis, and thus permitted a more 

 thorough study of their responses. The young of this species 

 are hatched in egg sacs as previously described. Upon the dis- 

 integration of these sacs the small leeches attach themselves by 

 their posterior suckers to the ventral surface of the parent leech. 

 In this position they are carried until they reach a considerable 

 size. I have not been able to determine just what is the cause 

 of their "weaning," though it no doubt is to be explained as 

 the resultant of two or three factors. One of these is the de- 

 creased thigmotaetic response of the young specimen as it be- 

 comes older. Another factor is a purely mechanical one; the 

 increase in size causes the young to become a serious impediment 

 to the progress of the parent, and the larger size makes the 

 burden easier to be rubbed off. Then, too, there is a limited 

 space for the carriage of young as the result of their increase 

 in size. This is obvious from the fact that the older the speci- 

 mens of young carried, usually the smaller the number of young 

 to be found attached. 



The formation of collections in the young, scarcely more than 

 one or two millimeters in length, when removed from the body 

 of the parent, is as clearly pronounced as we find it in the adult 

 glossiphonids. The formation of one of these collections is a 

 very interesting performance to observe. Some eight or ten 

 dozens of young of varying sizes, taken from several parents, 

 were placed one afternoon in a .small circular glass dish. By 

 the next morning they had grouped themselves into three collec- 

 tions in the angles of the dish. These groups were broken up 



