296 



PHYLUM ECHINODERMA 



bases of the tube-feet, but from each of these bases a canal ascends 

 between each pair of ambulacral ossicles, and expands into an ampulla 

 or reservoir on the dorsal or more internal side (see Fig. 160). The 

 fluid in the system may pass from the radial vessels into the tube-feet, 

 and from the tube-feet it can flow back, not into the radial vessel, but 

 into the ampullae. There are muscles on the walls of the tube-feet, 

 ampullae, and vessels. At the end of each arm there is a long unpaired 

 tube-foot, which seems to act as a tactile tentacle, and has also olfactory 

 significance. 



With regard to the vascular system there is considerable uncertainty. 



Fig. 160. — Diagrammatic cross-section of starfish arm. — 



After Ludwig. 



«., Radial nerve ; b.v., radial blood vessel according to Ludwig, 

 septum in pseud-hasmal vessel according to others ; w.v., 

 radial water vessel ; am., ampulla ; t.f., tube-foot ; p.c, a 

 pyloric caecum cut across ; s.p., a calcareous spine ; g., a skin- 

 gUl ; lac, spaces in the wall ; go., ova in ovary ; a.o.. ambu- 

 lacral ossicle. 



There is probably no definite vascular system at all. The organ 

 described as a heart is really the " genital stolon." There is a 

 " pseudhaemal sinus " surrounding the stone canal, leading into a 

 circumoesophageal ring, which gives off a vessel along each ray. 



From the dorsal surface and sides of a starfish in a pool, 

 numerous transparent processes may be seen hanging out 

 into the water. They are the simplest possible respiratory 

 structures, contractile outgrowths of the skin with cavities 

 continuous with the coelom, and are called " skin-gills," 



