THE STARFISH AND SOME ALLIES 249 



oral surface, aided by some of those on the aboral surface, 

 all extending and contracting much as in the starfish. The 

 uppermost tube-feet are used as tentacles for feeling, and it 

 may be for smelling also. The fact that sea-urchins collect 

 in numbers about the body of a large dead animal in the 

 water, indicates that they have the sense of smell. 



In certain parts of our eastern coast, members of the genus 

 Strongylocentrotus live in cavities in the rock, which are 

 excavated by the animals themselves. A related species living 

 on the coast of California burrows a hole in the solid rock deep 

 enough to conceal itself entirely. No one knows positively 

 how the burrowing is done. Probably three factors take part 

 to a varying degree, gnawing by the teeth, slow grinding 

 by voluntary movement of the spines, and incessant slight 

 turnings of the whole body by the waves. 



THE SEA-CUCUMBER 



Sea-cucumbers of various species are found in every ocean. 

 The species represented in Fig. 125 (Cucuma'ria chronhjel'mi) 

 is found in Puget Sound, Washington. It lives on the 

 bottom. 



The animals are about four inches long. The body is cylin- 

 drical, but without calcareous plates that touch one another 

 and keep the form constant. The only trace of a skeleton 

 comparable to that of the sea-urchin consists of scattered bits 

 of calcareous secretions of definite form beneath the skin. Al- 

 though the sea-cucumber is radially symmetrical, having the 

 five double rows of tube-feet along the cylinder, we can speak 

 of the anterior and posterior ends, because the animal lies flat. 



The anterior region is the oral region. At the centej; we 

 observe the round mouth, about which are the ten branching 

 tentacles. Internally there is a water-vascular system with 

 a ring-canal and radial canals. In respiration water is drawn 



