576 FRESH-WATER BIOLOGY 



The free-swimming members of the group have developed a 

 number of striking external peculiarities, due to differences in the 

 mode of life. Pterodina (Fig. 942) has a flat body, protected by a 

 hard cuticula forming a lorica; this shape aids it greatly in swim- 

 ming. Pedalion (Fig. 946) has developed six great limbs which Hke- 

 wise aid it in swimming. Similar Hmbs, but in a simpler condition, 

 are seen in Triarthra (Fig. 944) and Tetramastix (Fig. 945). In these 

 genera the function of the limbs seems to be mainly to protect the 

 animals from being swallowed by such predatory beasts as As- 

 planchna. One often sees an Asplanchna attempt to swallow one of 

 these at a gulp, but the prey at once extends its long appendages in 

 all directions, and these frustrate the attempt. The male of Pedalion 

 (Fig. 946, B) has simple appendages and bears a striking resemblance 

 to one of the simpler species of Triarthra (Fig. 944, B). 



An extraordinary offshoot of the Meticertidae is seen in the 

 spherical rotifer Trochosphaera (Fig. 947). In the corona, the jaws, 

 the lack of a foot, and various other features it agrees essentially 

 with the MeHcertidae, though its external form is very different. 



23-25. Bdelloida. This, the last group of rotifers, includes mainly 

 the genera Rotifer (Figs. 958, 960), Philodina (Fig. 959), Callidina 

 (Fig. 961), Microdina (Fig. 962), and Adineta (Fig. 957). They are 

 somewhat worm-Hke animals, often creeping like leeches, and found 

 in great numbers amid aquatic vegetation. They are specially 

 abundant in Sphagnum and other wet moss or moss-like plants; an 

 immense number of species particularly of Callidina are found in 

 such places. 



This group differs widely from the typical rotifers in many points. 

 The typical corona of the Bdelloida is a highly differentiated struc- 

 ture consisting mainly of two flat disks borne on stalks and with 

 cilia about their edges (Fig. 959, etc.). When the ciHa are in mo- 

 tion these two disks give the appearance of two revolving wheels. 

 It is to this that the name wheel-animalcule, and the Latin terms 

 rotifer and rotator are due; the Bdelloida were the first rotifers to 

 attract the attention of microscopists. The base of the stalks 

 bearing the disks is often clothed with short ciha. On the dorsal 

 side of the corona there is a long tentacle. 



The foot ends as a rule in three or four minute projections, by 



