THE PHYLUM ECHINODERMATA 



Fig. 16.5. Diagram of the am- 

 bulacra! system of Astenas. (Re- 

 drawn from W. R. Coe, 1912, 

 Geological and Natural History 

 Survey, State of Connecticut, 

 Bulletin 19.) 



Madreporite 



Tube foot 



process of respiratory exchange also. The madreporite, lying on the aboral 

 surface of the disk, contains many small openings into a tube, the stone canal, 

 which passes orally and joins a circular ring canal around the mouth. From 

 the ring canal a radial canal extends to the tip of each arm, passing just 

 above the radial nerve cord. At frequent intervals the radial canal gives 

 rise to short lateral canals, each of which terminates in a tube foot. At 

 its inner end each tube foot bears a muscular bulb, the ampulla,- the stalk of 

 the tube foot projects through the body wall, passing between the closely set 

 skeletal plates. The system as a whole contains a fluid which, like the 

 coelomic fluid, is practically identical with sea water. Circulation through 

 the system is maintained by the action of flagella on the lining epithelium. 

 The arrangement is such that the fluid contents of the system may pass freely 

 into each tube foot through the lateral canal, and within the tube foot may 

 flow back and forth between the stalk and the ampulla. Backflow from the 

 tube foot into the lateral canal is prevented by a valve-like structure. The 

 wall of the stalk contains longitudinal muscle fibers, and the terminal disk 

 or sucker is provided with muscle fibers whose contraction raises the central 

 portion of the disk to provide a vacuum for attachment to the substrate. The 

 longitudinal muscles are basally attached in a radial fashion to the adjacent 

 skeletal plates of the body wall. These so-called "postural muscles" pro- 

 vide for the pointing of the tube foot in any direction and thus allow directed 

 locomotion. The complex interactions of the musculature are reflexly co- 

 ordinated: contraction of the muscles of the ampulla forces fluid into the 

 stalk, causing it to extend in a direction determined by the state of the 

 postural muscles; the sucker is then placed on the substrate and attached by 

 contraction of its special muscles. As the body moves forward, the sucker is 

 released, and contraction of the longitudinal muscles of the stalk forces fluid 

 back into the ampulla and brings about retraction of the stalk. The cycle 



495 



