STRUCTURE OF ANNELIDA AND ENTOZOA. Ill 
tufts are arranged regularly on the several segments, and the 
animal can swim by the motion that it gives them; besides 
these, it has a kind of bristle-shaped appendage, that seems 
Fig. 52.— Nereis. 
like a rudimentary leg, which assists it in crawling. But 
there are others of these marine-worms, that form a tubular 
shell, in which they reside during the greatest part of their 
lives; and in these the gills, if disposed along the body, 
would have been removed from the access of water; they are 
therefore arranged round the head, often forming (as in the 
Serpulce , fig. 145) tufts of great brilliancy and elegance. 
105. Below the Annelida are other worm-like tribes of yet 
greater simplicity of conformation, but still presenting the 
same general plan of structure. Of one of these the common 
Leech may be taken as an example; of another, the Tape - 
Fig. 53.—Tape-worm. 
worm (fig. 53). This last belongs to a group termed Entozoa, 
from the circumstance that they inhabit the bodies of other 
animals. They are remarkable for the very low development 
of their digestive apparatus,—their nourishment being appa- 
