THE WORMS 59 
gether by elastic bands. A sac, the cocoon, is thus pro- 
duced, containing the eggs and a milky, nutritive substance. 
In a few weeks the worm 
develops and, bursting the 
wall of its prison, makes its 
escape. 
61. Distribution. — The 
earthworms and their allies 
are found widely distributed 
throughout the world, and 
all exhibit many of the 
characters just described. 
The greatest differences 
arise in their mode of life: 
some are truly earthworms, 
but others are fitted for a 
purely aquatic existence in 
fresh water or along the 
seacoast ; a few have taken 
up abodes in various ani- 
mals and plants, and in 
some of these situations they 
extend far up the sides of 
the higher mountains. In 
all, the head is relatively 
indistinct, the number of = 
bristles on each segment Fie. 36.—A marine worm (Nereis). A, ap- 
few, and for this and other —Praranes at Preeding season, and 3, 
reasons all are included in 
the subclass Oligochete, or “ few-bristle ” worms. 
62. Nereis and its allies—In many of the above-men- 
tioned situations members of a more extensive group of 
worms are found, with highly developed heads and many 
bristles arranged along the sides of the body. These are 
the Polychetes or ‘ many-bristle”” worms, and as a repre- 
sentative we may take Nere’s (Fig. 36), a very common 
