THE WHEEL ANIMALCULES (ROTATORIA) 567 



A further development of this line is seen in Polyarthra (Fig. 882). 

 Here powerful swimming organs have developed in the form of 

 appendages along the sides of the body, while the auricles have dis- 

 appeared. The animal never attaches itself, so that the disap- 

 pearance of the foot is complete. In Anarthra (Fig. 885) we find 

 precisely a Polyarthra that has not yet developed the appendages, 

 or that has lost them (?). 



Synchaeta and Polyarthra are typical open-water rotifers, consti- 

 tuting important elements of the plankton. 



To introduce the families of Rotifera next to be considered, it 

 is necessary to return to certain features of the Notommatidae. 

 Many of the species of that family show a very slight tendency to 

 a stiffening of the cuticula, so that the body retains a somewhat 

 definite form, often a little angular. Such notommatids are classed 

 in the genus Furcularia (Fig. 870). These are usually more active 

 than Proales or Notommata, and have longer, stiffer toes. By 

 accentuation of these features of Furcularia, and by further spe- 

 cialization, there are formed several families of free-swimming 

 rotifers: 



3. Salpinidae. The cuticula becomes more hardened, and three 

 or four longitudinal furrows are formed, one in the dorsal middle 

 line, one on each side, and sometimes a weak one in the ventral 

 middle line. Thus there is produced a sort of armor or lorica, 

 composed of three or four plates (Figs. 886, 887). Such loricas are 

 seen in most pronounced form in Salpina (Fig. 886). But every 

 possible intermediate gradation exists, leading from Furcularia to 

 Salpina. The intermediate steps are mostly classed in the genus 

 Diaschiza (Fig. 887) ; here the cuticula is only slightly stiffened, and 

 the longitudinal clefts are little marked. The species of Diaschiza 

 are many of them hardly distinguishable from Furcularia or even 

 from Notommata; they were formerly classed in these two genera. 

 In Salpina the lorica is strongly developed and bears long spines 

 or teeth. Diplois and Diplax stand between Diaschiza and 

 Salpina, having strong loricas but no teeth. There is thus a 

 continuous series from the Notommatidae to Salpina. The Sal- 

 pinidae are common amid vegetation. (See the Monograph on 

 Diaschiza by Dixon-Nuttall and Freeman, 1903.) 



