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 asa heart is really the ‘‘ genital stolon.” There is a ‘‘ pseud- 
heemal 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 
L, Td. 
a bv. 
I, 
Fic. 134.—Diagrammatic cross-section of starfish arm.— 
After Ludwig. 
#., Radial nerve; J.v., radial blood vessel according to Ludwig, 
septum in pseud-hezmal vessel according to others; w.v., 
radial water vessel; az., ampulla; 7, tube-foot; g.c., a 
pyloric caecum cut across ; s.f., a calcareous spine; g., askin- 
gill; Zac., spaces in the wall; go., ova in ovary ; @.0., ambu- 
Tacral 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 
