THE WHEEL ANIMALCULES (ROTATORIA) 571 



much developed, so that they stand high above the general surface 

 of the corona (Fig. 923). They partly enclose a sort of funnel, 

 open on the ventral side, which leads down to the mouth. In 

 most species of Brachionus the integument has become very thick 

 and hard, so as to form a stout lorica, often bearing spines or teeth 

 (Fig. 921). But Brachionus mollis Hempel (Fig. 925) marks the 

 transition in this respect, the integument being merely a little 

 stiffened and without spines or teeth. In Brachionus, as in Ploesoma 

 and Gastropus, the stout foot is marked with rings. The jaws are 

 constructed on much the same plan throughout all these groups. 



The Brachionidae are among the most numerous of the rotifers 

 found in ponds and pools amid vegetation. Some of the species 

 are extremely variable. 



14. Anuraeidae. An offshoot of the Brachionidae is found in the 

 Anuraeidae (Figs. 913, 916). The general organization is the same 

 as in Brachionus, but the foot has been lost, though in the males 

 (Fig. 913, C) it is retained. The lorica shows in some species of 

 Notholca a tendency to run into bizarre forms (Fig. 916). The 

 Anuraeidae are among the commonest of the rotifers of the plank- 

 ton; they vary extremely with seasonal and other changes. 



15. Asplanchnidae. The group diverging by way of Hydatina 

 is now left, and another offshoot of the Notommatidae taken up. 

 In the Asplanchnidae the body remains soft, but becomes large and 

 inflated, while the foot disappears; the jaws are of a remarkable 

 type known as theincudate (Fig. 929, B\ and the alimentary canal 

 loses its posterior opening (see Fig. 929, A\ the undigested waste 

 being disgorged through the mouth. But one finds in all these 

 respects forms transitional between the Notommatidae and the 

 Asplanchnidae. Thus, Asplanchnopus (Fig. 927) retains the foot, 

 though it lacks the intestine, and has the characteristic jaws of this 

 family. Harringia (Fig. 928) retains not merely the foot, but like- 

 wise the intestine. Its corona is like that of As plane hna while its 

 jaws (Fig. 928, B) are squarely intermediate between the usual form 

 and the incudate type characteristic of Asplanchna. The typical 

 incudate jaws consist mainly of the very large incus (fulcrum and 

 rami), the mallei having nearly or quite disappeared; but in Har- 

 ringia all the typical parts of the jaws are clearly seen. 



