230 ECHINODERMA. 



Integument. (a) The body is covered by a ciliated ectoderm. This 

 includes supporting, glandular, and sensory cells, and beneath it 

 there is a network of nerve fibrils with ganglionic cells. 



(b) The middle layer of the integument consists of a double stratum 



of ground substance, the outer part of which contains the chief 

 limy structures except the ambulacra! ossicles which are formed 

 more internally. There is also a thin muscular layer. The 

 whole of this middle layer is formed in development from the 

 mesenchyme tissue. 



(c) Internally the body wall is lined by a ciliated epithelium, derived 



in development from the wall of the ccelomic pouches. (See 

 Development.) 



Between two of the arms lies the perforated madreporic plate, the 

 entrance to the water vascular system, thus defining the bivium, 

 while the other three arms constitute the trivium. 



The Calcareous Skeleton. 



In association with the inner mesodermic layer of the 

 integument, there is developed on the ventral surface of each 

 arm a double series of sloping plates. These two series 

 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 modified 

 spines, known as pedicellarice, look like snapping scissor 

 blades mounted on a single soft handle. They have been 

 seen gripping Algae and the like, and probably keep the sur- 

 face of the starfish clean. 



Muscular System. 



A starfish is not very muscular, but it often bends its arms 

 upwards by means of the muscular layer noted above, and 

 may sometimes be seen tightly embracing an oyster. 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. 



