SEA URCHINS AND SAND DOLLARS. 



Sea Urchins and Sand Dollars would 

 scarcely seem, at first sight, to be re- 

 lated to the starfishes, yet they are in 

 many respects identical in structure. In 

 the Sea Urchin, the five rays of the star- 

 fish are rolled together to form a spheri- 

 cal or flattened disk, which is covered 

 with spines of greater or lesser length. 

 These spines work in a ball-and-socket 

 joint and are capable of considerable 

 movement. The intestine in the Sea 

 Urchins is unusually long and is twisted 

 about in the shell in a manner similar to 

 that of the human intestine. This great 

 length is necessary to accommodate the 

 large quantities of sand which the Sea 

 Urchin swallows to obtain the minute an- 

 imals which it contains. 



In one respect the Sea Urchins are 

 unique, viz., in the possession of a pe- 

 culiar dental apparatus known as ''Aris- 

 totle's Lantern." This is composed of 

 five teeth which are operated by a com- 

 plicated system of muscles. When ex- 

 amined closely, this apparatus is seen to 

 be made up of five triangular pieces 

 which terminate in a sharp tooth, each 

 one being reinforced on the inside by a 

 heavy rib. With these teeth the Sea 

 Urchin is able to dig up quantities of 

 sand containing minute foraminifers and 

 to browse along the rocks which are cov- 

 ered with marine vegetation. 



Unlike the starfish, the Sea Urchin is 

 a lover of home, for we find him scoop- 

 ing out great holes in the rocks along 

 the shore in which he lives most of the 

 time, leaving them only when in search 

 of food. The animals love the seclusion 

 afforded by cracks and holes in the rocky 

 shores, and even cover themselves with 

 seaweed, which they pull about them with 

 their tentacles. This instinct of protec- 

 tive adaptation is found in many 

 branches of the animal kingdom and is 

 in many cases the means of saving the 

 species from complete extinction. 



The spines of the Sea Urchin are sup- 

 posed to be principally for protection 

 against its enemies, but in some species 

 they are an efficient aid to locomotion, 

 assisting the ambulacra, or feet, and en- 

 abling the Sea Urchin to progress much 

 more rapidly than by means of the am- 

 bulacra alone. Arbacia punctulata, a 

 common Sea Urchin of the New England 

 coast, uses its spines for this purpose and 

 by a tilting motion advances quite rap- 

 idly, the wear and tear on the spines of 

 the lower surface modifying them to a 

 spathiform shape. 



In addition to the spines and feet 

 there occur scattered over the surface a 

 number of pincer-like organs, known as 

 pedicellarise. They are composed of a 

 stem which becomes enlarged near the 

 middle and terminates in a knob-shaped 

 apparatus composed of three parts like a 

 three-pronged fork arranged concentri- 

 cally ; they are jointed, and open and shut 

 at the will of the animal. When closed 

 the forks appear to form but a single 

 acorn-shaped knob, so closely do they fit 

 together. 



The general office of these pedicellarise 

 is not certainly known, but one of their 

 functions is the removal of the waste 

 products of digestion. The rejected 

 particle is received by one of these little 

 pincers and passed by this to the next 

 pincer; this is repeated until the particle 

 reaches the edge of the urchin, when it 

 is dropped off into the water. One may 

 see this rejected matter passing rapidly 

 along the lines upon which the pedicel- 

 larias occur in greatest number, looking 

 like miniature roads for carrying off the 

 waste products. 



One of the most abundant of the Sea 

 Urchins is the common green sea-egg or 

 sea-porcupine (Strongylocentrotus dro- 

 bachiensis), which occurs on both the 

 New England and the Californian coast. 

 It is strictly a rock lover and its holes 



179 



