622 FRESH-WATER BIOLOGY 



"head," a narrower part just behind it called the "neck" and the 

 larger "rump," or body proper, which constitutes the major portion 

 of the animal. These regions are not sharply limited and sometimes 

 can not be distinguished at all. They do not correspond to any 

 internal structures. The ventral surface is more or less flattened 

 and the dorsal surface arched as is conspicuously seen in side view. 



The head bears at its tip an opening, the mouth, surrounded by 

 a row of deHcate oral bristles which point forward. The sides of 

 the head are often lobed and carry two circles or series of groups 

 of fine sense hairs; those of the anterior series point outward and 

 forward while the others are usually directed backward. Some- 

 times the entire body is smooth, or it may be partly or entirely 

 covered with plates, spines, or hooked bristles. These furnish 

 the criteria for the distinction of species and are carefully described 

 in the key. 



The posterior end of the body may be bluntly rounded, pointed, 

 or forked. The caudal processes, often spoken of as "toes," carry 

 special bristles and contain cement glands, the secretion of which 

 is expressed through terminal pores and enables the animal to 

 attach itself temporarily to objects in the water. 



On the ventral surface are two bands of cilia near the median 

 line, extending nearly the entire length of the body. These con- 

 stitute the chief organs of locomotion. The movements of the 

 Gastrotricha are so graceful as to elicit admiration from every 

 observer. In motion they recall the long-necked infusoria, though 

 excelling the latter in speed and variety of movement. By bend- 

 ing the body sharply on itself the animal may instantly reverse its 

 course. Those species possessed of long bristles utilize them in 

 moving by leaps like jumping rotifers. Other species employ the 

 caudal processes in movement, looping the body and attaching the 

 tips of these toes successively in different places. 



The internal anatomy is simple. From the mouth (Fig. 964, A), 

 a straight ahmentary canal traverses the length of the animal 

 terminating in a simple anal orifice just above the posterior end of 

 the body. One can distinguish an anterior muscular region, the 

 esophagus and a posterior portion, the intestine (Fig. 964, C), which 

 is lined by large digestive cells, rich in protoplasm. Small gland 



