VIII HABITS 227 



Other forms again are parasitic, Proales wemechii is found 

 in Vaucheria, a coarse, dark green, thread-like Alga found in fresh 

 water; and the closely allied P.parasita is not uncommon in the 

 beautiful floating green spheres of Volvox. 1 Albertia, Drilo- 

 phagus, and Balatro are parasitic on or in fresh-water Oligochaetes ; 

 the curious Seisonaceae are parasitic on Ncbalia, a small Crusta- 

 cean easily obtained in masses of whelk's eggs ; the aberrant 

 Bdelloid Discopus attaches itself to the surface of the Holo- 

 thurian Synapta. Similarly among this last Order Calliclina 

 parasitica attaches itself to the limbs of the fresh-water Crustacea 

 Gammarus and Asellus. These are rather commensals than true 

 parasites. The species of Brachionus often attach ' themselves 

 temporarily to the common water-flea Daphnia. 



Besides a few Ploima, the vast majority of the Bclelloicls live 

 in or among mosses and their roots. Many Calliclina inhabit 

 cup-like hollows in the leaves of the scale mosses (Jungerman- 

 niaceae), especially of the genus Frullania. Almost all the 

 members of this Order are susceptible of desiccation and revival ; 

 certain species, such as Rotifer vulgaris, Philodina roseola, 

 Adineta vaga, etc, can be readily obtained by moistening gutter 

 dust. The mechanism of the process is as follows : when desicca- 

 tion is gradual the animals close up their telescopic bodies and 

 excrete gelatinous plugs at either end, which effectually seal 

 them against further drying ; if, however, they be dried on a 

 slide without any debris, the process is too rapid for them to 

 protect themselves, and they therefore die. This was dimly seen 

 by others, and clearly demonstrated by H. Davis, 2 who records 

 the following experiment : The Bev. E. J. Holloway, having found 

 Philodina roseola in gutters, placed strips of paper there in the 

 rainy season, and succeeded in obtaining clean gatherings, taking 

 dry groups of a hundred together, having a varnish- like covering 

 all over ; and being glued to one another, mostly in one plane, 

 and to the paper, forming a pavement. In the dry condi- 

 tion they resist extremes of temperature ; thus Zelinka found 



1 I have recently found a large species of this genus dwelling in the shell of the 

 large Cladoceran Crustacean, Euryccrcus lamellatus. It is remarkable for its power 

 of completely telescoping its extremities within the middle segments, and for its 

 immense foot-glands, both characters being doubtless correlated with its habitat. 

 Kousselet identifies it with P. pdromyzon. 



2 Month. Micr. Journ. vol. ix. 1873, p. 287 ; Journ. Quekett Club, ser. 2, vol. 

 ii. 1884-86, p. 231. 



