110 _ STRUCTURE OF MYRIAPODA AND ANNELIDA. 
through a series of metamorphoses, in which they lose their 
eyes and legs, and become fixed for the remainder of their 
lives. 
103. We now pass back to another class of the higher 
group of Articulata, adapted to breathe air and to inhabit 
the land,—the Myriapopa or Centipede tribe (fig. 42). Both 
these names are derived from the great number of legs 
possessed by these animals, which often amount to 60 pairs 
or even more. In this class we see a more perfect equality 
of the segments or divisions of the body than in any others 
among the higher Articulata; and the similarity is scarcely 
less complete in the internal arrangement, than it is in the 
external form. In its lower tribes (fig. 51), the legs are so 
Fig. 51.—Ivuxvus. 
weak as scarcely to be able to sustain the body, which moves, 
therefore, partly in the manner of that of a worm. The 
animals of this class undergo no proper metamorphosis ; but 
there is a considerable addition to the number of their seg- 
ments and legs after they have come forth from the egg. 
104. We now pass to the lower division of Articulata, in 
which the body possesses no jointed members ; and the animals 
belonging to this group are for the most part included in the 
class of ANNELIDA, the Leech and Worm tribe. We here find 
the body enveloped,—not in a hard casing, formed of distinct 
pieces united by a flexible membrane,—but in a skin which 
is altogether flexible, and which gives little indication of a 
division into segments. This class includes several distinct 
tribes, which all agree, however, in the long worm-like form 
of the body, and in the similarity of the different ganglia or 
their nervous system. The Zarth-worm and its allies are 
adapted to live on land and to breathe air; but the greater 
number of Annelids are purely.aquatic ; and these breathe 
by gills, which form tufts that are disposed on various parts 
of the body. In the Nereis, or Sea-centipede (fig. 52), these 
