THE WHEEL ANIMALCULES (ROTATORIA) 



575 



the typical free-swimming rotifers. Yet again, as in most other 

 cases, free-swimming species form a transition to these extreme 

 types. One finds the same peculiar corona, the same remarkable 

 type of jaws, and various other features in common with the 

 Melicertidae, in a number of free-swimming rotifers. These in- 

 clude the genera Pterodina (Fig. 942), Pompholyx, Pedalion (Fig. 946), 



FIG. 865. Corona of Lacinularia socialis Ehr., to show the two wreaths of cilia. A, Dorsal view. 

 B, Side view. (After Wesenberg-Lund.) 



Triarthra (Fig. 944), and Tetramastix (Fig. 945). These rotifers are. 

 the only ones that have corona and jaws like those of the Meli- 

 certidae, and they agree with them in many other particulars. 

 Thus, all have two eyes, while most other rotifers have but one. 

 In all there is either no foot, or it is a peculiar one, lacking the 

 characteristic toes. In Pterodina the foot ends in a bundle of 

 cilia, and this is likewise true of the young of the Melicertidae. In 



FIG. 866. Malleo-ramate jaws. A, Jaws of Melicerta ringens Schrank. (After Weber.) B, Jaws 

 of Pterodina caeca Parsons. (After Rousselet.) 



many Melicertidae there is below the mouth a peculiar fold of in- 

 tegument forming the so-called "chin" which plays a part in the 

 formation of the pellets used for building the tubes. This chin 

 is likewise found, in a slightly less developed condition, in Pedal- 

 ion (Fig. 946, ch) and in Triarthra, while nothing of the sort is 

 found outside the present group. The remarkable similarity of 

 corona, jaws, eyes, and other features seems to demonstrate clearly 

 that all these free-swimming rotifers are closely related to each 

 other and to the Melicertidae. 



