ASTEROIDEA. 255 



well-defined simple arms containing the gonads and prolonga- 

 tions of the gut, and with a ventral ambulacral groove 

 supported by paired ossicles and bearing the tube-feet ; with 

 regularly disposed calcareous, often spinous, plates on the skin ; 

 with an external madreporite (occasionally multiple), always 

 on the upper surface of the disc in living forms ; with a 

 mouth at the centre of the lower surface^ and usually with an 

 anus at the opposite pole. 



Description of a Starfish. 



The description applies especially to the common five- 

 rayed starfish (Asterias or Asteracanthion rubens). It is 

 often seen in shore pools exposed at low water, but its 

 haunts are on the floor of the sea at greater depths. There 

 it moves about sluggishly by means of its tube-feet. 



Each of the five arms bears a deep ventral groove in 

 which the tube-feet are lodged. The mouth is in the 

 middle of the ventral surface, the food canal ends about 

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

 form, the 11-13 rayed sun-star (Solastcr\ the pincushion- 

 like Porania, 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 

 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 

 adambulacrah) 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 pedicellaricE, look like snapping 

 scissor-blades mounted on a single soft handle. They 



