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ZOOLOGY 
arranged in two rows on the ventral surface of each arm, 
and not in four, as appears to be the case in Asterias. 
Their tube-feet have pointed extremities, and not a sucking- 
disk. 
Besides the ampullae on the radial vessels, additional 
Fic. 184.—Solaster papposus (upper surface). 
reservoirs for the water-vascular fluid usually occur on the 
circumoral ring; these are termed /Polian vesicles, and are 
usually five or ten in number. It is doubtful whether the 
vesicles which occur near the right position in Asterias rubens 
are really Polian vesicles, that is, opening into the ring, or 
whether they are the first pair of ampullae of each radial 
vessel. In one species, Cribella oculata, some of the openings 
in the madreporic plate lead into that section of the body- 
cavity which surrounds the heart and stone canal, instead of 
into the latter canal. 
The Asteroidea have great powers of regenerating lost 
parts. Arms broken off grow out again from the disk, and 
even the whole disk may be regenerated from a single 
separated arm. 
