256 PHYLUM ECHINODERMA. 
have been seen gripping Alge and the like, and probably 
keep the surface of the star-fish clean. . 
A starfish is not very muscular, but it often bends its 
arms upwards by means of a muscular layer in the body 
wall. . Other muscles affect the size of the ventral grooves, 
and muscular elements also occur on the protrusible part 
of the stomach, and in connection with the water vascular 
system. 
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Fic. 132.—Starfish. 
I. Ventral surface; 4.4, tube-feet extended; a.g., the ambulacral 
groove with the tube-feet retracted; 2., the mouth. II. Dorsal 
surface, showing the position of the madrepore (J7.); the two 
adjacent arms form the bivium. 
Underneath the ciliated ectoderm lies a network of 
nerve fibrils, with some ganglion cells. But besides these 
diffuse clements there is a pentagon around the mouth, 
and a nerve along each arm. The system is not separable 
from the skin. Ganglion cells are developed also on certain 
parts of the wall of the ccelom. 
A red eye spot, sensitive to light, lies on the terminal 
ossicle at the tip of each arm, and is usually upturned. It 
is a modified tentacle, bearing numerous little cups, lined 
