394 ECHINOZOA. 



" sand-canal," which opens into the circular vessel surrounding the 

 gullet. From the circular canal arise radiating ambulacral vessels, 

 which correspond in number with the number of the arms, and 

 which are lodged, along with the radiating nerves, in deep "am- 

 bulacral grooves " on the under surface of the arms. Each radi- 

 ating vessel gives off two or four rows of cylindrical tube-feet or 

 "pedicels," the ends of which are usually sucker-like, and which are 

 used in locomotion. The tube-feet are protruded by means of 

 vesicles or "ampullae," which spring from their bases, and are situ- 

 ated superiorly to the radiating vessel. As compared with the 

 Echinoids, the essential peculiarity of the ambulacral system is that 

 the radiating ambulacral vessels are situated externally, and are not 

 covered over by a calcified integument. There are, therefore, no 

 structures in the Asteroids which can be compared with the per- 

 forated ambulacral plates of the Sea-urchins, and the tube-feet of 

 the former do not pass through any portion of the integumentary 

 skeleton on the way to the surface. On the other hand, the radi- 

 ating ambulacral vessels of the Star-fishes are protected by an inter- 

 nal skeleton, which is not present in the Echinoids, or which is only 

 imperfectly represented in some types of the latter by the so-called 

 "auriculae." This internal skeleton has the form of a double 



Fig. 272. — Diagrammatic section of the ray of Asterias rubeus. a a, Ambulacral ossicles ; 

 b, Position of the ambulacral vessel ; c c, Plates of the external skeleton ; n, Nerve-cord. The 

 dotted lines show the tube-feet proceeding from the ambulacral vessel, but the ampullae are not 

 represented. 



series of elongated calcareous plates, the so-called " ambulacral 

 ossicles," which form the roof of the ambulacral groove on the 

 under side of each arm, and are so apposed to one another as to 

 form a kind of elongated pent-house, beneath which is placed the 

 radiating ambulacral vessel (fig. 272, a a). The ambulacral ossicles 

 on the one side of the ambulacral groove may be directly apposed 

 to the ossicles of the corresponding row on the other side of the 

 groove (as in the typical Star-fishes), or the ossicles of the one row 

 may alternate with those of the opposite row (as in the ancient group 

 of the Encrinasterid). In either case the ambulacral ossicles are 



