THE WORMS 61 
are developed in almost every segment, whence they are 
finally swept to the exterior through the tubes of the kid- 
neys (Fig. 37, B). 
The Nereis and its immediate relatives are all active 
forms, and by means of powerful jaws, which may be quickly 
extended from the lower part of the mouth cavity, they 
capture large numbers of small crustaceans, mollusks, and 
worms which happen in their path. Others more distantly 
related make their diet of seaweed, and 
many living on the sea bottom swallow 
great quantities of sand, from which they 
absorb the nutritious substances. 
63. Sedentary forms.—Preyed upon by 
many enemies, a large number of species 
have been forced to abandon an active ex- 
istence save in their early youth, and to 
construct many interesting devices for their 
protection. Numerous species, shortly after 
they commence to shift for themselves, 
build about their bodies tubes of lime (Fig. 
39), from which they may emerge to gather 
food and into which they may dash in times 
of danger. As the worm grows the tube is 
correspondingly enlarged, and these tubes, 
in all stages of construction and variously At 
coiled, may be found on almost every avail- yg. 3g common 
able spot at the seashore, and may often marine worm (Po- 
be seen on the shells of oysters in the {Hiv .ctenaci mos 
markets. boscis and over- 
In other species the tube is like thin peer one hal 
horn, and may be further strengthened or 
concealed by numerous pebbles, bits of carefully selected 
seaweeds, or highly tinted shells, which give them a very 
attractive appearance. Such species usually develop out 
of immediate contact with other forms, but a few live 
so closely associated together that their twisted tubes 
