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 circular like disks. Sometimes they 
are cut at intervals on the margin; again they have slits through 
the body (lunales). 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 ECHINANTHIDE 
Genus Clypeaster 
_ Cc. ravenellii. 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 
