THE SEA URCHINS 



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current through the slits, or they may remain and undergo direct development 

 in the pouches themselves. Around the mouth are a number of short flat processes 

 or papillae, serving as strainers, and keeping foreign bodies that are not wanted for 

 food from entering the stomach. Round the mouth are also twenty tentacles, which 

 are really the modified tube feet of the first two arm segments of each arm. They 

 are in^ a state of continual movement, assisting the food to enter, and clearing 

 away the undigested residue, which is ejected from the mouth. 



The branched ophiurids or Cladophiurce are sedentary, attaching themselves 

 by coiling their branching arms around corals and suchlike animals; but can move 

 when they please. The same mode of life is also affected by a few of the simpler 

 forms; but, as a rule, ophiurids have considerable powers of locomotion, of which 

 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 

 flexuous 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 Echinoidea 



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 



TEST OF EDIBLE SEA URCHIN, WITH THE SPINES 

 REMOVED FROM THE LEFT-HAND HALF. 



(Natural size.) 



