PHYLUM ECHINODERMATA 233 



to extend them. The walls of both ampullae and tube feet are mus- 

 cular. In large starfish the tube feet may be extended an inch or 

 two. The sucker ends of these tube feet work like a vacuum cup 

 and will adhere effectively to surfaces over which the animal is 

 drawing itself. When the pressure is released by the ampulla, the 

 tube foot contracts and draws the animal forward. When water 

 is again forced into the tube, it releases its grip and is again 

 extended. By alternation of the activity of tube feet in different 

 parts of the body the animal is able to move itself from one place 

 to another. The entire water vascular system is a modified part 

 of the coelom. 



A thin-walled system of vessels running parallel to the water 

 vascular is the circulatory system. It is enclosed in a perihemal 

 space. In addition to this the coelomic fluid, which occupies the 

 coelom and bathes all of the organs, serves as a circulatory medium 

 in that it absorbs the digested food and distributes it. This fluid 

 bears amoebocytes which are cells capable of picking up particles 

 of waste material and carrying them to the dermal branchiae, where 

 they pass through the membrane to the exterior. These dermal 

 branchiae are pouches of the coelomic wall which extend outward 

 between the skeletal plates and have the additional function of 

 respiration. When these pouches are completely extended, they 

 nearly cover the exterior surface of the animal, and thus expose 

 an enormous area to the water for respiration. 



Excretion is carried out in part by the amoebocytes which have 

 been produced by the Tiedemanu's bodies and have migrated to 

 the coelomic cavity. The rectal caeca serve in respiration to some 

 extent also. There is a certain amount of diffusion of dissolved 

 wastes through the dermal branchiae and the walls of the tube feet. 



The nervous system is radially arranged about the oral ring which 

 encircles the mouth just orally to the ring canal. From the oral 

 ring, a radial nerve extends the length of each arm and ends in the 

 pigmented eyespot. These nerves lie in the roof of the ambulacral 

 grooves. The aboral surface is supplied by a less conspicuous aboral 



a, anus ; ac, anterior coelom ; ad, anterodorsal arm ; b, brachiolar arms ; ci, 

 adoral ciliated band ; dr, dorsal surface developing rays ; es, esophagus ; f, point of 

 fixation ; int, intestine ; I, lateral arm ; Ic, left coelomic pouch ; m, mouth ; md, 

 median dorsal arm ; mp, madreporic pore and water tube ; pad, posterodorsal arm ; 

 po, postoral ciliated band ; pr, preoral ciliated band ; re, right coelomic pouch ; sp, 

 spines; st, stomach; w. five water tubes of the water vascular system. (Modified 

 from Wilson and McBrlde. By permission, The Macmillan Co.) 



