124 



Echinoderma. 



Fig. 76. Pedioellarise of a 

 Sea-urchin closed and open. Proxi- 

 mal portion of tlie stalk not 

 drawn. — Orig. 



Size. The tube-feet which end in suckers^ may serve as locomotor organs, 

 since they can stretch out and attach themselves to foreign objects, and 



then contract so as to draw the body after 

 them; when they are rounded at the 

 ends, they usually have a tactile \f unc- 

 tion. Within the feet there are cavities 

 connected with the water-vascular 

 system peculiar to the Echinoderms. 



The water-vascular system, 

 a series of tubes, containing a fluid, and 

 lined with a ciliated epithelium, consists 

 of a ring-canal surrounding the 

 alimentary canal close to the mouth, and 

 of five branched radial canals in 

 connection with it. The ring-canal, which 

 is usually beset with a number of vesi- 

 cular outgrowths (polian vesicles), com- 

 municates with the exterior by the so- 

 called stone-canal,* which is at- 

 tached to a perforated plate, the 

 madreporite, lying in the body-wall, and allowing of the passage 

 of water. The radial canals lie along the middle of each radius 



giving ofE a tiny vessel, provided 

 with a small swelling or ampulla, 

 to each tube-foot. Water is driven 

 into the tube-feet by the contrac- 

 tion of the water-vessels and 

 ampuUee, and causes their elonga- 

 tion ; when, however, they contract, 

 it is driven back into the canals.t 

 In most Holothurians, and in 

 Crinoids, the stone-canal (or canals, 

 for there may be several) is not 

 connected with the surface, but 

 opens into the body-cavity by one 

 or more apertures, through which 

 the fluid is taken into the water- 

 vascular systein. In the Crinoids, 

 the body-wall is perforated by fine 

 pores, through which the sea-water 

 passes into the body-cavity. 



The watei'-vasoular system of the lai-va 

 is always in direct commimication with 

 the sea-water by a stone-canal opening 

 at the surface. Here, too, there is always only a single stone-canal. 



* This name is derived from the fact that the wall of the canal often contains calcareous 

 deposits. .j- Amoeboid cells, like those of blood, float in the water-vascnlar fluid. 



Piif. 77. ' Diagrammatic sketch of the 

 water-vascular system of a Star- 

 fish, ap ampulla, h ring - canal, ma 

 madreporite, j) polian vesicle, r radial 

 canal, s tube feet, st stone canal. — Modified 

 from Gegenbaur. 



