HABITS 227 



Other forms again are parasitic. Proales werneckii 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 Nebalia, 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 Callidina 

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

 Gammarus and Asellus. These are rather commensals than true 

 parasites. The species of Brctchionus often attach themselves 

 temporarily to the common water-flea Daphnia. 



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

 in or among mosses and their roots. Many Callidina 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 Rev. 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, Eurycercus 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. 

 Rousselet identifies it with P. petromyzon. 



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

 ii. 1884-86, p. 231. 



