566 FRESH-WATER BIOLOGY 



1. Notommatidae. It will be helpful first to notice some of the 

 chief variations of type among the Notommatidae themselves. 

 The simplest, most undifferentiated rotifiers that exist are those 

 conmionly classed in the genus Proales. They have small, soft 

 bodies, nearly cylindrical, and obscurely segmented externally 

 (Fig. 856). The foot and toes are short. The corona is a uniformly 

 ciliated, nearly plane surface on the ventral side and anterior end 

 (Fig. 859). These rotifiers are smaU, sluggish creatures, very numer- 

 ous, but not differing greatly among themselves, so that the species 

 are hard to distinguish and students of the rotifiers have paid 

 Uttle attention to them. In other species of the Notommatidae 

 the corona has become differentiated in a pecuHar way, forming 

 the so-called auricles; these species are classed mainly in the genus 

 Notommata. The auricles are portions of the ciliated area set off 

 prominently on each side of the corona and bearing stronger ciha 

 (Fig. 857, B) ; they serve to enable the animal to move more rapidly. 

 In the simplest cases the auricles are directly continuous with the 

 rest of the cihated disk, as in Notommata aurita (Fig. 878) . In other 

 cases there is a space without cilia between the disk and the auri- 

 cles (Fig. 881). The auricles are commonly kept contracted when 

 the animal is creeping about, so that their existence would not be 

 suspected. But when the animal prepares to swim through the 

 water it unfurls these auricles and sails away. The species of 

 Notommata are more active than Proales, and there are greater 

 differences among the different members of the genus. 



2. Synchaetidae. A Kne of divergence, consisting essentially in a 

 greater development of those characteristics of Notommata which 

 give it rapidity of movement, leads to the production of what 

 is commonly classed as a different family, — the Synchaetidae 

 (Fig. 880). In Synchaeta the entire corona is very large, 

 occupying the large end of the cone-shaped body, while the 

 auricles are highly developed, forming powerful swimming organs 

 which are set off at a distance from the remainder of the co- 

 rona. By the aid of these auricles the species of Synchaeta dash 

 about with such rapidity that they can hardly be followed with the 

 microscope. (See the monographic study of the Synchaetidae by 

 Rousselet, 1902.) 



