3 o8 



E CHINODERMS. 



they readily avail themselves. Most walk rather than creep, raising themselves 

 on their five arms as upon legs ; stretching out one or two arms in front, and 

 drawing the rest of their body in the same direction. Even in a state of repose, 

 the arms alone touch the ground, the disc remaining raised above it In other 

 forms, however, the rays of the body undulate laterally, and produce a creeping 

 serpentine movement. Rondelet wrote that the common brittle-star creeps by the 

 nexuous movement of its rays in the manner of serpents, and, placed on dry land, 

 never ceases to move them, until it casts them off in pieces, which, although 

 separate, move by bendings, like parts of worms and the cut-off tails of lizards. 

 The little Amphiura, which lives under stones, among the roots of seaweed, 

 can turn its arms very quickly around its disc, and so form itself into a little 

 ball ; thus, if it be disturbed, it can roll and sink quickly into deeper parts of the 

 water. Sometimes ophiurids are seen to progress by a kind of rowing motion of 

 the arms. 



The Sea-Urchins, — Class Bchinoidea. 



The sea-urchins are the best known, as they are the most numerous of all 

 echinoderms. The annexed illustration shows the test or shell of the egg-urchin, 

 with the spines on the right side, but scraped away from the left. The plates of 

 the test are seen to be covered with rounded tubercles of various sizes, and it is to 



these that the spines are attached by 

 a ball-and-socket joint, surrounded by 

 muscles that can move the spines in 

 any direction. The tubercles do not, 

 however, cover the whole test indis- 

 criminately, but are disposed chiefly in 

 five broad zones, extending from one 

 pole to the other. Alternating with 

 these are five narrower zones, bearing 

 smaller and fewer tubercles, and pierced 

 by small holes arranged in regular rows. 

 Through these holes pass the tube-feet, 

 which are all provided with suckers 

 at the end. These latter zones are, 

 therefore, the ambulacral zones ; one 

 of them being seen in the middle of the illustration. The other zones are called 

 interambulacral, and one of them is shown on the left of the same illustration. All 

 the zones converge towards the summit of the test, where the vent is situated 

 in a circular space covered with membrane. This membrane contains a few 

 irregular granules, and is surrounded by five large interradially placed plates, 

 pierced by the ducts of the generative glands. One is also pierced by a large 

 number of small water-pores, and is called the madreporite. Outside these five 

 plates, and alternating with them, are five other plates, each situated at the top of 

 an ambulacral zone, and pierced by the unpaired tentacles, which terminate the 

 water-canals, and represent the unpaired tentacles near the eye at the ends of the 

 arms of a star-fish. At the other pole of the body is another membrane, sur- 



TEST OF EDIBLE SEA-URCHIN, WITH THE SPINES REMOVED 

 FROM THE LEFT-HAND HALF (nat. size). 



