224 MARINE INVERTEBRATES 



GENUS Echinus 



E. gracilis. This is a deep-water species, which occurs from Cape 

 Cod southward. The shell is nearly spherical, but is a little depressed 

 on the oral side, and has twenty bands of color, alternately green and 

 white. The spines are short and thin. 



GENUS Toxopneustes 



T. variegatus. This is the common species of the Southern States, 

 from North Carolina southward, and is found in shallow water in pro- 

 tected places. The shell is nearly globular ; the spines vary in thickness 

 and color, some being long, slender, and greenish, while others have 

 stout and blunt spines of a yellowish or violet tint. 



ORDER CLYPEASTROIDEA 



CAKE-URCHINS 



The animals of this order, commonly known as sand-cakes or 

 sand-dollars, are flat and circTilar like disks. Sometimes they 

 are cut at intervals on the margin ; again they have slits through 

 the body (limales). They have a well-marked star-shaped figure 

 on the dorsal surface. This figure is formed by the ambulacra, 

 or tube-feet, which run in five rounded or petal-shaped lines on 

 the under surface. The mouth, in Aristotle's lantern, is in the 

 center of the somewhat concave ventral surface, the petal-like 

 ambulacral zones meeting at the central space. The excretory 

 opening is on the margin of the disk, at a point between two of 

 the sections of the star-like figure. This marks the posterior part 

 of the body, while the opposite arm of the star marks the front 

 or anterior end. The spines are very fine and silky, and are 

 spread abundantly over the whole animal. Clypeasteroids are 

 mainly found in sand considerably below low- water mark, though 

 some species thrive where they are exposed to the surf on open 

 sandy beaches. 



FAMILY ECHDTANTHIDJE 

 GENUS Clypeaster 



C. ravetiettii. This species occurs in deep water from South Caro- 

 lina southward. The disk is about four inches in diameter, and is raised 

 in the center into a large cone. The ambulacra run down the sides of 



