570 FRESH-WATER BIOLOGY 



body of the Notonunatidae, but approaching the definite permanent 

 form found in Ploesoma. The corona (Fig. 912, B) is much like that 

 of Hydatina, save that the fine cihation of the general surface has 

 disappeared. Notops pelagicus (Fig. 910) shows a further step in 

 the same direction; the cuticula is here stiffened to form a thin 

 transparent lorica, of sufficient stiffness to form angles and teeth, 

 though with by no means the thickness and soHdity found in 

 Ploesoma and Brachionus. 



From Notops pelagicus it is but a short step in one direction to 

 Gastropus and Ploesoma, in another to Brachionus. 



10. Gastropodidae. The transition from Notops to Gastropus is 

 shown hy Gastropus hyptopus (Fig. 915), which was originally con- 

 sidered a species of Notops, and which if it stood by itself would 

 still be placed in that genus. The lorica is here soft, the body 

 short and thick. The lorica becomes more marked, and the other 

 peculiarities more pronounced in the other species of Gastropus, 

 Gastropus stylifer (Fig. 917) forming the extreme in this direc- 

 tion. 



11. Anapodidae. Probably derived from forms similar to Gas- 

 tropus by a process of reduction are the species of Anapus (Fig. 911), 

 in which the foot is lacking, the corona smaU and simple. 



12. Ploesomidae. The species of P/oe^oma (Figs. 918 to 920) are 

 closely related to Notops and Gastropus. Ploesoma truncatum (Fig. 

 920) shows a lorica only a Kttle stronger than that of Notops pel- 

 agicus, and resembhng that of Gastropus hyptopus, though it has 

 many irregular wrinkles. In other species of Ploesoma the lorica 

 becomes stronger and marked in very pecuhar ways. Ploesoma 

 lenticulare (Fig. 918) and P. hudsoni (Fig. 919) are among the most 

 active and powerful of the predaceous Rotifera. They tear their 

 way through the water at a furious rate, darting from side to side, 

 and seizing and devouring with their powerful jaws other rotifers 

 with which they come in contact. The Ploesomidae are among the 

 most important plankton organisms. 



13. Brachionidae. From Hydatina and Notops to Brachionus 

 the step is perhaps still shorter than to Gastropus and Ploesoma. 

 In Brachionus (Figs. 922, 923) the three prominences that surround 

 the mouth in Hydatina and Notops (see Figs. 906, 910) have become 



