ASTEROIDEA. 



183 



which prevents the fluid passing back into the radial canal, so 

 that when the ampulla contracts, it drives its contents into and 

 so extends the tube-foot. The retraction of the foot is caused 

 by the contraction of the longitudinal muscles of its walls, the 

 fluid being driven back into the ampulla. If the tube-foot has 

 a sucker, it is able to be attached to external objects and then 

 by its contraction to draw the body of the starfish towards the 

 object. Of course movement of the body in this way can only 

 be effected when a number of feet are acting together in a co- 

 ordinated manner. If a starfish be removed violently from its 

 substratum, the attached sucker will be broken off and left on 

 the stone and water will be ejected from the lacerated ends of 

 the contracting tube -feet. 



FIG. 134. Diagrams of transverse sections through the stone-canals of various Asteroids (after 

 Lang). 1 membrane by which the stone-canal is attached to the wall of the axial sinus ; 

 2 epithelium of the axial sinus ; 3 epithelium of stone-canal ; 4 connective tissue of wall 

 of stone-canal. 



The stone-canal projects into the axial sinus (p. 180). On one side of 

 it, viz. on the side opposite that by which it is attached to the wall of 

 the axial sinus (Fig. 130), there is a longitudinally disposed fold of its 

 lining membrane. In the simplest cases this fold projects into the canal 

 as a ridge (Echinaster purpureus, Brisinga coronata (Fig. 134, A). In 

 other forms (Asterina gibbosa Penn, Cribrella oculata Linck, etc.) the 

 free edge of the fold splits into two lamellae (Fig. 134, B), which in yet 

 other species (of Asterias, Pentaceros, Gymnasterid, etc.) become coiled 

 (C). A further complication is introduced by the fusion of the ridge 

 with the opposite wall of the canal and the formation, from each surface 

 of the septum so constituted, of a spirally coiled lamella (species of Astro- 

 pecten, etc., D). Finally there are forms (Astropecten aurantiacus, species of 

 Luidia and Culcita)in which these septa are present in great number and 

 divide the whole lumen into many irregular chambers. All these ridges 

 and lamellae vanish at the lower end of the canal, where it joins the 

 circular vessel. The walls of the stone-canal and the lamellae, etc., 

 which project into it, contain a strong deposit of calcareous matter. 



