726 THE AMERICAN NATURALIST. [Vor. XXXV. 
This frequently ends in two small points, called the toes. 
There may or may not be a distinct head, set off from the 
body by a neck. Dorsal and ventral surfaces are usually (not 
always) markedly differentiated. In many rotifers (Loricata) 
the cuticula or outer covering of the body is hardened to form 
a shell or Zorica ; this may bear elevations, spines, teeth, etc. 
The simplest and probably primitive form of the corona is 
that of a plane disk covered with equal cilia, on the ventral side 
of the animal at the anterior end, as in Proales. From this the 
varied forms found have probably been derived, chiefly by the 
following steps : (1) the outer cilia became longer and stronger, 
forming a distinct marginal wreath; (2) the cilia within this 
‘wreath were partly or entirely lost; (3) a second wreath of 
cilia was developed (or has remained from the original cilia) 
within or without the above-mentioned wreath. Thus we 
obtain the condition, which has often been considered the 
primitive one, of two concentric wreaths, with the mouth 
between them. This condition is found only in much special- 
ized forms. 
In the alimentary canal can be distinguished a mouth, usu- 
ally situated excentrically in the coronal disk and leading into 
a muscular pharynx, or mastax, which is furnished with chiti- 
nous jaws or ¢rophi; a narrow cesophagus; a large stomach 
furnished with a pair of gastric glands, and an intestine leading 
to the anus. The latter is lacking in some groups. The jaws 
or trophi are very various in form and furnish a most important 
systematic character. Certain types of jaws are distinguished ; 
of these, the more important are the malleate (Fig. 170) and 
the forcipate (Fig. 171). The other forms (ramate, Fig. 34: 
incudate, Fig. 54; uncinate, Fig. 10, etc.) may be related to 
one of these two types. 
Points of especial practical importance for distinguishing 
species are the following : form, structure, and position of the 
corona; presence or absence, form and position, of the foot ; 
form of the toes; presence, number, and position of the red 
eye-spots ; structure of the jaws or trophi ; presence or absence 
of the intestine and anus ; nature of the cuticula, — whether 
soft and flexible or hardened to form a shell orlorica ; presence 
