SEA-URCHINS. 



171 



pores upon the oral surface are either scattered widely over the ambulacral and some- 

 times over the inter-ambulacral plates, forming pore arece, or they are arranged in 

 bands which ramify over both the ambulacral and the interambulacral plates. The 

 difference between the anterior and posterior extremity is well marked in this order, 

 the former being known by the odd ambulacrum of the three ambulacra composing 

 the trivium, the latter by the position of the anus between the posterior pair of ambu- 

 lacra or bivium. 



The jaws of the elypeastroids are much simj^ler than those of the regular echini, 

 and articulate upon the auricles, instead of being held in place by muscles, as in the 

 latter. They are V-shaped, and are placed horizontally. The teeth are secured in a 

 groove corresponding to the line of junction of the arms of the V. The spines are 

 in all cases delicate and short, often almost velvety in their fineness. The water system 

 of the elypeastroids is without Polian vessels, but there are large Yesicles at the 

 bases of the suckers. The species mostly live uj)on sandy or muddy bottoms. 



In the EucLYPEASTEiD^ the upper and lower floors of the test are connected and 

 strengthened by pillars, needles, or radiating partitions of calcareous matter. Schi- 

 nocyamus picsillus and a few other species are almost as globular as the Echinidse, 

 but are true Clypeastroids in structure, with simple partitions extending inwards from 

 the circumference. In Clypeaster and Echinanthus the floors are connected by jsillars, 

 slender in the former genus, massive in the latter. The sj)ecies are large, and, though 

 flattened, have a rounded ambitus, while the oral surface is concave. C. rotundus 

 occurs on the west coast of North Amei-ica, as far north as San Diego, while Echi- 

 nanthus rosaceus is tolerably common about the West India Islands and Florida. 

 Laganum and its near relatives have the floors connected by walls that run parallel 

 to the edge of the test. 



In the ScuTELLiD^ the test is extremely flat, and is usually more or less circular. 

 The great quantity of calcareous matter forming the flattened edge is in many species 



lessened somewhat by the presence of 



cuts or openings in the ambulacral or 



interambulacral areas. The furrows 



of the under surface, which are straight 



in Clypeaster, are in this group more 



or less branching, and the upper and 



lower floors are supported by parti- 

 tions that radiate from single j)oints. 

 Some of the best known forms of 



Scutellidffi belong to the genus Echi- 



narachnius, and are without cuts or 



lunales. E. parma, the Sand Dollar, 



is found on the Atlantic coast of the 



United States, and also on the Pacific 



coast as far south as Vancouver Island, 



and in Asia as far as Jaj)an. E. ex- 



centricus is the common cake-urchin 



of the Pacific Coast, from Monterey 



northward, and occurs also in Kamts- 



chatka. It is very common in San Francisco Bay, where it lives upon the sand at 



a depth of five to seven fathoms. 



Fig, 150. — Ecldnarachnius parma, sand-dollar. 



