78 



ANIMAL STUDIES 



are developed in almost every segment, whence they are 

 finally swept to the exterior through the tubes of the kid- 

 neys (Fig. 43, 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. 



73. 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. 

 45), 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 

 coiled, may be found on almost every avail- 

 able spot at the seashore, and may often 

 be seen on the shells of oysters in the 

 markets. 



In other species the tube is like thin 

 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 



FIG. 44. A common 

 marine worm (Po- 

 lynoe brevisetosa), 

 with extended pro- 

 boscis and over- 

 lapping plates cov- 

 ering the back. 



