Phylum Echinodermafa and Related Groups 543 



The ivater vascular system is a unique feature of the members of 

 this phylum. This is a series of tubes filled with fluid and connected to 

 numerous tiny ambulatory structures, the tube feet. The water enters 

 this system through the madreporite on the aboral surface. The madre- 

 porite is connected by the stone canal to a ring canal around the mouth. 

 From the ring canal, other passageways carry the fluid to the rows of 

 tube feet. In the starfish, the tube feet are arranged in rows along 

 the' oral surface of the arms. These rows are in grooves known as the 

 ambula^ral grooves. In other echinoderms, the arrangement differs 

 slightly due to the difference of body form, but is essentially the same. 

 Each tiny tube foot is a complex organ consisting of a saclike 

 ampulla at one end, a hollow connecting tube, and a tiny sucker disc at 

 the other end. When fluid is forced from the ampulla by contraction 

 into the distal portion of the tube foot, the sucker disc expands and 

 flattens against any surface. When the fluid is withdrawn, the disc is 

 flaccid. Because there are so many of these tube feet, the echinoderms 

 are able to exert great force. 



The circulatory system is poorly developed among the echinoderms. 

 There is a ring canal above the mouth with various radial canals ex- 

 tending out from it. There is no central heart and only coelomic fluid 

 circulates in the vessels. These fluids contain amoebocytes. Respira- 

 tion is carried on in various ways. Among some the coelomic fluid is 

 exposed through special structures to the oxygen-laden sea water; 

 among others the water is brought into special respiratory devices. 

 Among the first group are the dermal branchiae of the starfish which are 

 tiny outpouchings of the body cavity exposed through openings in the 

 skeleton. Among the latter type is the respiratory tree of the sea cu- 

 cumber. There is no special excretory system, rather waste products 

 are excreted by the amoebocytes which wander to the gills or intestinal 

 wall and unload their waste materials. 



A ring around the mouth with radial nerves running along the 

 ambulacral grooves constitutes the nervous system. Other nerves 

 branch from these radial nerves to the tube feet and other parts of the 

 body. Under the epithelium is a dense network of nerves which seem 

 to originate from a nerve ring around the anus. There are few sense 

 organs among these animals although they do respond quickly to 

 changes in their environment. The epithelium appears to be greatly 

 sensitive and among some starfishes there are light sensitive spots at 

 the tips of the arms. Some of the sea cucumbers have statocysts. 



