SOME AQUATIC WORMS, ETC. 1 79 



to form what may be called the head, a slight constriction 

 behind this part constituting the neck; the broadened 

 central portion of the body is formed with convex 

 lateral borders and a inore or less strongly arched back 

 or dorsum, this region being variously appendaged 

 with spines or with scales, and suddenly narrowed to 

 produce the posterior extremity, which is forked, and 

 bears two conspicuous but short tail-like prolongations. 

 The lower or ventral surface is a flat and nearly level 

 plane extending the entire length of the body. It 

 bears one longitudinal band of cilia near each lateral 

 border, seldom more. The head is usually somewhat 

 triangular, and formed of three or of five rounded 

 lobes, and has two tufts of vibratile hairs on each side. 



The mouth is on the ventral surface of the head, and 

 under a moderate amplification seems to be a circular 

 opening, but with an objective of high power it will 

 be found to be beaded and somewhat complicated in 

 structurfe, as shown in Fig. 135. 



The whole upper surface of the body is, in the dif- 

 ferent species, covered with rounded papillse, scales, 

 spines, or prickles, or with both scales and spines at 

 the same time. In the latter kinds the' scales cover 

 the back, and sides, and the spines spring from these 

 appendages, arching back towards the forked tail. 

 And in all cases these little scales are imbricated, like 

 the shingles on a roof, only they have the peculiar 

 habit of overlapping in what seems to be the wrong 

 way, because their free margin, or that region which 

 represents their free margin, points toward the animal's 

 head, or in a direction exactly opposite to that of the 

 scales of a fish. They are usually minute, and require 

 high powers to show them properly. 



