STRUCTURE OF STARFISH 



293 



scissor-blades mounted on a single soft handle. They 

 have been seen gripping Algae and the like, and probably 

 keep the surface of the starfish 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. 



Fig. 158. — Starfish. 



I. Ventral surface ; t.f., tube-feet extended ; a.g., the ambulacral groove with the 

 tube-feet retracted ; m., the mouth. II. Dorsal surface, showing the position 

 of the madreporite (M.) ; the two adjacent arms form the bivium. 



Underneath the ciliated ectoderm lies a network of 

 nerve fibres, with some ganglion cells. But besides these 

 diffuse elements 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 coelom. 



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 tube-foot, bearing numerous little cups, lined 



