where waves break over it ami the sun is particularly 

 bright, Lytechinus often uses the tube-feet on its 

 aboral surface to hold pieces of seaweed and bits of 

 coral or stones as a shield and shade. In deeper wa- 

 ter this habit is less frequent. 



Echinus iniliaris, found in British coastal waters, 

 tends to conceal itself in the same way. Apparently it 

 is a less active animal than its close relative, the edi- 

 ble urchin E. esciilentiis (Plate 136), for the latter 

 remains clean as it forages about for a mixed diet of 

 shellfish, tube-building worms, crustaceans, small 

 echinoderms, and hydroids. 



The "sea egg" of Barbados and other islands in 

 the West Indies is Tripueiistes ventricosiis. a particu- 

 larly common urchin sought for human food at sea- 

 sons when the orange-colored ovaries are loaded 

 with eggs. Native people collect large numbers of 

 them, break them open, and either eat them raw or 

 roast them on the half shell or fry the ovaries as 

 though they were an omelet of hen's eggs. In Italy the 

 egg masses of sea urchins are marketed in coastal 

 towns as "frutta di mare." The favored Mediterra- 

 nean species is Paracentrotiis lividus. 



Recent immigrants to New England sometimes 

 seek out the largest specimens of a green sea urchin, 

 Strongylocentrotiis droehhachiensis (recipient of one 

 of the longest scientific names on record). It takes 

 the place of Arbacia north of Cape Cod. It is found 

 also on northern European and Pacific coasts. This 

 animal is like Arbacia in being mostly a vegetarian, 

 feeding on seaweeds of definite kinds. Along the Ca- 

 nadian east coast, it has become addicted to a diet of 

 cannery wastes. In the Baltic Sea, it often varies its 

 diet with hydroids, tube-building worms, and other 

 foods. The big purple or red Strongylocentrotiis 

 franciscaniis (Plate 137) is sought by Italians in 

 California for its tasty ovaries, which are eaten raw. 



A fair number of different urchins bore into rock, 

 seemingly by working on it with the hard teeth of the 

 Aristotle's lantern or by abrading it with spines. An- 

 other possibility is that they keep the rock so free of 

 plant particles that erosion is hastened. Any loose 

 particles are probably removed by the tube-feet. 

 In any case the process is slow. 



The commonest boring urchin along rocky coasts 

 from Norway and Iceland to the Cape Verde Islands 

 is Psammecliinus miliaris. In the Mediterranean and 

 farther down the west coast of Africa, Paracentrotiis 

 lividus has the same habit, and can be found in 

 honeycombed rock — a dark green animal with 

 spines of bright green, violet, and brown. Strongy- 

 locentrotus purpuratiis. a purple urchin along the 

 Pacific side of North America, not only cuts cavities 

 in hard rock but has done extensive damage to steel 

 posts used as wharf pilings in California. Its food 

 consists mostly of plant materials. 



If one of these boring urchins is disturbed, it at- 



tempts to wedge itself at the bottom of its cavity. 

 Possibly this is its protection against wave action too. 

 Yet when the tide is in these animals apparently 

 wander away from their holes, feed on algae or other 

 material, and then return to the security of the 

 home they have prepared. Sometimes an urchin be- 

 comes imprisoned in its cavity, having opened a big 

 enough room but not enlarged the doorway through 

 which it entered at a smaller size and younger age. 



On rocks of various Pacific islands, Colobocen- 

 trotus atratus demonstrates a very different technique 

 in resisting wave action. Its aboral spines are all 

 short, flat, bladelike organs that shield the body from 

 debris carried in the surf. Similar spines around the 

 somewhat flattened body suggest the petals of a 

 daisy. These too seem to aid the animal by using the 

 force of a wave to hold the body against a rock. 



Heterocentrotus inainmillutus is the slate-pencil ur- 

 chin of the Indo-Pacific and Hawaii. Its slightly flat- 

 tened spines may be Vi of an inch in diameter and 

 5 inches long. The lime of which they are composed 

 is hard and white; it can be used to make clear, 

 erasible marks on old-fashioned writing slates. 



THE CAKE URCHINS AND 

 SAND DOLLARS 



A bit of broken shell from a cake urchin or a sand 

 dollar shows many diflferences from any fragment of 

 a sea urchin's test. The limy plates are thicker and 

 little vertical struts (such as no sea urchin possesses) 

 extend as braces between the aboral and oral sur- 

 faces. 



The intact shell of a cake urchin or a sand dollar 

 shows, too, a bilateral symmetry through slight elon- 

 gation and in the displacement of the anal opening 

 toward the edge of the shell, on either the oral or the 

 aboral surface. The aboral surface itself bears a strik- 

 ing pattern of five petal-shaped marks (petaloids) 

 corresponding to the tracts of tube-foot holes in a sea 

 urchin's shell. 



Over much of the oral and aboral surfaces of both 

 cake urchins and sand dollars, short tube-feet extend 

 singly through a multitude of small openings. Along 

 with inconspicuous pedicellariae they serve to keep 

 the body clean and perhaps also in feeding. Those 

 on the undersurface aid the spines in locomotion and 

 in the digging movements by means of which these 

 animals sink themselves in the surface sediments. 



Cake urchins are oval creatures with no distinct 

 edge to the shell. The common Clypeaster is cov- 

 ered by a dense, furlike coating of short dark-brown 

 spines. C. rosaceus is known as a "sea biscuit" in the 

 West Indies. C. suhdepressus burrows shallowly in 

 tidal areas from North Carolina to Brazil. 



Sand dollars are very flat, the edge of the body 

 thin and distinct. Most of those known live along 

 sandy shores of America and Japan. Echinarachnius 



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