teeth that come together toward the outside and the 

 center of the oral surface. Aristotle, who discovered 

 this organ in the fourth century B.C., described it as 

 resembUng "a horn hintern with the panes of horn 

 left out," and it has been called "Aristotle's lantern" 

 ever since. With it a sea urchin can chew a wide 

 variety of foods and possibly also excavate living 

 spaces in rocky shores. 



So great a range of different echinoids inhabits the 

 coasts of the Indian Ocean and Malaya that those 

 areas are regarded as the world center for shore- 

 inhabiting kinds. In all directions from that center the 

 number of unlike types of echinoids decreases. It 

 shrinks too in progressively deeper water, and be- 

 low fifteen thousand feet none is known. Temperate 

 and polar seas have the largest number of individuals, 

 whereas in the tropics, communities of urchins are 

 less frequent although the number of different kinds 

 is more impressive. In the Arctic, urchins often con- 

 gregate in such abundance that it would be impos- 

 sible for a skin diver to set down a foot between one 

 urchin and the next. 



About 750 species of living echinoids have been 

 identified, most of them members of groups with rep- 

 resentatives in shallow water. Some of these can be 

 recognized at a glance. 



SEA URCHINS 



Hatpin urchins, the bane of waders and skin div- 

 ers, are the most respected echinoderms in tropical 

 and subtropical waters. They include also the largest 

 of the regular echinoids to be found near shore. 

 Much larger ones, which belong to family Echino- 

 thuridae. are found in very deep waters. 



The spines of these urchins may be 1 foot long, 

 shaped like needles, jet black, fragile, hollow and 

 probably poison-filled. They penetrate human skin 

 easily, break off', and cause intense stinging pain. 

 Eventually the lime of the spine is absorbed. Whorls 

 of minute teeth around each spine resist extraction, 

 and the material of the spine itself tends to crumble 

 in a pair of tweezers. 



No one who has watched these big urchins on a 

 reef or experimented with them in an aquarium 

 tank has any doubts about the role of the long spines. 

 The urchin keeps them in constant motion, and re- 

 sponds to any shadow by turning even more spines 

 in that direction. With only a general sensitivity to 

 light in the black skin covering the shell, the urchin 

 seems very well aware of any change in its surround- 

 ings affecting the illumination falling upon it. 



The hatpin urchin of the Mediterranean and tropi- 

 cal eastern Atlantic is CeiUro.stepluiiuis longispinus, 

 a black-bodied animal with brightly colored spines 

 kept constantly in motion, the tip of each spine trac- 

 ing a small circle in the water. Diculema setosum of 

 the East Indies and D. antillarum of the West Indies 



and Florida keys present the same formidable appear- 

 ance. Commonly they cluster in cavities of coral reefs, 

 and all of the really large ones seem to have protec- 

 tion of this sort. 



When stirred into movement a hatpin urchin can 

 travel at from one to one ;;nd one-half inches per 

 second, "walking" on the tips of shorter spines over 

 the oral surface. This is about sixty times as fast as 

 the maximum for the common sea urchins of New 

 England coasts on their multiple tube-feet. 



Cidarid urchins differ in that they bear two very 

 different sizes and types of spines. The large ones 

 may be as long as the diameter of the shell, and are 

 widely spaced and covered by a wooly, hairlike ma- 

 terial to which foreign particles often cling. Their 

 small spines, by contrast, are as spotlessly clean as 

 those of other sea urchins. Some of the small spines 

 form a whorl around the base of each large spine. 



Of cidarids. Cidaris tribuloides is familiar in tropi- 

 cal parts of the eastern Atlantic, and from North 

 Carolina to Brazil, throughout the West Indies, and 

 in Bermuda. It reaches a diameter of IV4 inches and 

 is mottled in various shades of brown. Often its large 

 spines carry such a crust of moss animals (bryo- 

 zoans) that the bands of purplish red and yellow are 

 concealed. 



In scientific circles the sea urchins that have be- 

 come most distinguished are plain purplish brown, 

 measuring between 1 and 2 inches in diameter, with 

 the anus in the middle of the aboral surface clearly 

 equipped with four or five large plates acting as 

 valves. These urchins are poorly armed, usually 

 lacking glandular pedicellariae and bearing only mod- 

 erately slender spines about half as long as the width 

 of the shell. Shorter spines around the mouth wear 

 shiny caps. 



These urchins of the genus Aibacia have provided 

 experimental biology with study material to a de- 

 gree paralleled only by the fruit fly in genetics, the 

 white rat in nutrition, and the frog in investigations 

 of muscle action. The most famous of them is Aibacia 

 piinctiilata. found from Cape Cod to Florida and 

 throughout the West Indies. A. li.xula lives in the 

 Mediterranean and along tropical coasts of the east- 

 ern Atlantic. A. stellata is a very similar urchin oc- 

 curring from Mexico's Baja California to Peru along 

 the eastern Pacific. 



Most of the familiar sea urchins are not hatpin 

 urchins or cidarids or distinguished members of 

 the genus Arbacia. Instead, they are of types with 

 solid spines and an abundance of all four types of 

 pedicellariae. 



On shores from the Carolinas through the West In- 

 dies, the good-sized, somewhat flattened urchin 

 whose solid, white spines against a darker body give 

 it a shaggy appearance, usually proves to be Lyte- 

 chinus variegatus. Close to the limit of low tide, 



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