ASTEROIDEA. 



259 



With regard to the vascular system there is considerable uncertainty 

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

 scribed as a heart is really the "genital stolon." There is a "pseud- 

 haemal sinus" surrounding the stone canal, leading into a circum- 

 cesophageal 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 



FIG. 134. Diagrammatic cross-section of starfish arm. 

 After Ludwig. 



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

 septum in pseud-haemal vessel according to others ; tv.v., 

 radial water vessel ; am., ampulla; /._/"., tube-foot; ^>.c., a 

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

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

 lacrnl ossicle. 



continuous with the ccelom, and are called "skin-gills." 

 It is likely that pigmented cells of the body cavity fluid act 

 like rudimentary red blood corpuscles ; the water vascular 

 system may help in aeration; and the whole body is, of 

 course, continually washed with water. 



The " skin-gills " are said to have an excretory function ; 

 for phagocytes, bearing waste, seem to traverse their walls. 

 It may also be that excretion is somehow concerned in 



