226 ECHINODERMA. 



the centre of the dorsal disc. With this flat, five-rayed 

 form, the n-13 rayed sun-star {Solaster), the pincushion- 

 like Goniaster, and the flat pentagonal Palmipes, should be 

 contrasted. Between two of the arms lies the perforated 

 madreporic plate, thus defining the bivium, while the three 

 other arms constitute the trivium. 



The body is covered by a ciliated ectoderm, beneath 

 which lies a mesodermic layer. In association with the 

 latter there is developed on the ventral surface of each arm 

 a double series of sloping plates. These meet dorsally, like 

 rafters, in the middle line of the arm, forming an elongated 

 shed. The rafter-like plates are called ambulacral ossicles ; 

 the groove which they bound lodges the nerve-cord, the 

 blood vessel, the water vessel, and the tube-feet of each 

 arm. 



In association with the outer mesodermic layer of the 

 integument, numerous smaller plates are developed, e.g. the 

 adambulacrals, which articulate with the outer lower ends of 

 ambulacrals. The dorsal surface bears a network of little 

 ossicles, and many of these bear spines. Peculiarly modi- 

 fied spines, known as pedicellaria, look like snapping scissor- 

 blades mounted on a single soft handle. They have been 

 seen gripping Algse and the like, and probably keep the 

 surface of the star-fish clean. 



A star-fish 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. 



Underneath the ciliated ectoderm lies a network of nerve 

 fibrils, with some ganglionic 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. 



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 

 by sensitive and pigmented cells, containing clear fluid, and 

 covered by cuticle. The skin is diffusely sensitive. The 

 terminal tube-foot of each ray seems to be olfactory. 



