ZOOGENESIS 



mollusks in possessing a jointed and usually much 

 elongate body in which each segment is, to a greater 

 or lesser degree, a semi-independent unit. 



While many of the jointed worms live swimming 

 freely in the water or crawling on the surface of sub- 

 merged objects, they are especially adapted to a bur- 

 rowing life, either boring through mud or winding 

 their way through the canals in sponges or through 

 crack and crevices in rocks, in coral heads, or in any 

 other objects. Their slender bodies, which are usu- 

 ally soft and extremely flexible and are made up of a 

 more or less long series of semi-independent units, 

 fit them for a manner of life impracticable for the vast 

 majority of the vertebrates, the arthropods, or the 

 mollusks. 



The echinoderms, which include the starfishes (fig. 

 42., p. 71), brittle-stars (fig. 44, p. 87), sea-urchins 

 (fig. 41, p. 71), feather-stars (fig. 43, p. 87) and sea- 

 lilies (fig. 6, p. 5) and their allies, and the sea- 

 cucumbers or holothurians, are peculiar in having 

 when their final form is reached a radial symmetry 

 with almost invariably five divisions of the body, a 

 usually heavy outer — though not properly external — 

 skeleton which is composed of pavement-like or re- 

 ticulated or vertebra-like plates and is wholly unlike 

 the jointed external skeleton of the arthropods, and a 

 curious hydraulic system called the water-vascular 

 system by means of which they operate their tube- 

 feet, which are their locomotor organs, and com- 

 parable and other structures. 



The heavy outer armor of the echinoderms, which 



