232 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 



Fxo. 134. — Solasier papposus (upper surface). 



reservoirs for the water-vascular fluid usually occur on the 

 circumorai 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 ruhens 

 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, Crihella 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. 



