XVI. THE STARFISH AND ITS ALLIES 



Stricture of a Star fish. \— A g^nce at the body of a starfish shows us 

 that the name is rightly given. The body is called the disk; the five racU- 

 ating structures the arms or rays. The term echinoderm (leaning spinv- 

 us^ll sir^^^Thesl^elt^^^ ^^ examination of the dried specim/n b^fo?e 

 the animal is literally em- 

 bedded in the skin and pro- 

 trudes as many thousands of 

 little spines. Closer examina- 

 tion reveals the fact that the 

 skeleton is composed of very 

 many tiny plates, all articu- 

 lated together in such a way 

 as to give great flexibility as 

 well as strength to the frame- 

 work of the starfish. 



Notice that the thin skin 

 covers the skeleton, but that 

 in a ray broken in cross sec- 

 tion the interior of the ray is 

 hollow. In the starfish the 

 arms have the position of radii 

 of a circle; hence the animal 

 is said to be radially symmet- 

 rical. 



Notice the differences be- 

 tween the dorsal and ventral 

 surfaces. The latter is called 

 the oral surface because of the 

 position of the mouth, which 

 can be seen as a hole in the 

 center of the disk. In living 

 specimens the baglike stomach 

 is frequently found projecting 

 from this hole. 



The five grooves, which lead outward along the rays from the area around 

 the mouth, are known as the amhulacral grooves because they contain the 

 ambulacroB or tube feet. In the dried specimen the tube feet may be found 

 as very small dried projections in the grooves. There are four rows of 

 tube feet in each groove. Estimate the number of tube feet in a single row 

 and thus figure out the number of tube feet in a starfish. Do you believe 

 the number to be exactly the same for every starfish? Give reasons for 

 your answer. Locomotion in the starfish is performed by the movement 

 of hundreds of the little suckerlike feet. The process of movement of a 

 single tube foot is a complicated one. It is performed partly by means 

 of muscles, but chiefly by means of the passage of water through a system 

 of water tubes within the body of the animal. 



* See Hunter and Valentine, Manvxd, page 147. 



203 



Ventral or under surface of the starfish. The dark 

 circle in the middle is the mouth, from which 

 radiate the five ambulacral grooves, each filled 

 with four rows of tube feet. Photograph half 

 natural size, by Davison. 



