STARFISH AND OTHER ECHINODERMS 



367 



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 ex- 

 tended. 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 



Fig. 139. — Starfish "walking" on glass. Viewed from the oral surface. Notice the 

 extended tube feet. (Courtesy of General Biological Supply House.) 



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

 bears amoehocytes 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 Tiedemann's bodies and have migrated to 

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



