2l8 Merkel Henry Jacobs 



cellular alga, Spaehrella lacustris (Haematococcus pluvialis), on 

 which it feeds and from which it appears to derive the red color 

 which gives it its specific name. In size, it is microscopic, being 

 barely visible as a minute speck to the naked eye. Adult individ- 

 uals when fully extended measure from 0.35 to 0.5 mm. in length 

 and when contracted from o.i to 0.15 mm. The newly hatched 

 young are about five-eighths as long as the adults which they 

 resemble closely except in being colorless and in having a more 

 transparent alimentary tract. 



The members of the group Bdelloida are peculiar among the 

 rotifers in their method of locomotion. In addition to their swim- 

 ming movements which resemble those of other rotifers, they are 

 able to creep over the surface of solid objects like a leech, hence 

 the name applied to them. These creeping movements are ren- 

 dered possible by the telescopic structure of the body, which may 

 readily be extended or contracted, and by the presence at its ante- 

 rior end of a so-called proboscis by means of which adhesion to 

 solid objects is possible. The proboscis of Philodina is fairly stout 

 and when the trochal disc has been withdrawn within the body, as 

 it always is when the animal is creeping, it appears to form the 

 anterior end of the body. When the trochal disc, which consists 

 c f two ciliated lobes which the earlier observers mistook for wheels, 

 is extended, the proboscis is drawn back so as to lie behind and 

 dorsal to it. In creeping, the animal first fastens its foot by means 

 ( f a sticky substance secreted by the cement glands contained in 

 it; the proboscis is then attached and the foot drawn forward, the 

 body shortening and arching itself, like that of a leech or a ''meas- 

 uring worm." The foot is then again attached and the body 

 extended. These movements may occur with considerable rapid- 

 ity although the progress of the animal is not so fast as when swim- 

 ming. As a rule a high temperature and fresh water favor 

 swimming movements and low temperatures and foul water creep- 

 ing movements. 



Externally Philodina is covered with a fairly thick cuticle which 

 in the regions of the head and foot is divided into a number of seg- 

 ments by alternate stiff and flexible portions. By an inroHing 

 of the flexible regions the stiffer parts are allowed to fit within one 



