86 



SYSTEMATIC ZOOLOGY 



ambulacral feet, and constitute the chief organ of locomotion. 

 In the skin calcareous spicules are developed, which may unite 

 to form larger or smaller plates, sometimes distinct from one 

 another as in the sea cucumbers, often united to form a tolerably 

 firm skeleton, like that of the common starfishes, of which the 

 parts may be movable upon each other to a certain extent, or 

 perfectly rigid as in the sea urchins. 



In their internal structure the Echinodermata are strikingly 

 different from the Ccelenterata. There is no coelenteron, but, 



instead, several distinct cavities 

 within the body. The mouth 

 opens into a gastric or digestive 

 cavity which usually terminates 

 in an anus. A well-marked 

 body-cavity, such as has alreadv 

 been described as characteristic 

 of the Metazoa, is present, and 

 this contains the principal or- 

 gans of the body, such as the 

 digestive system and the repro- 

 ductive organs. Further, there 

 is a system of blood vessels, 

 constituting the vascular cavity ; 

 and finally, a peculiar system 

 of tubes, which is found in no 

 other animal type and known 

 as the water-vascular or ambu- 

 lacral system, for it is connected 

 with the chief organs of loco- 

 motion. This last system merits 

 a more extended description ; 

 it consists of several parts (Fig. 74). On the surface of the 

 body, opposite the mouth, i.e. the aboral surface, there is gen- 

 erally a skeletal plate situated excentrically in an interradius, 

 pierced by numerous fine holes, a sievelike structure, called the 

 madreporic plate, because of its resemblance in many starfishes 

 to corals of a similar name. This plate connects with a canal 

 which extends from the aboral side of the animal toward the 

 oral, and called the stone canal, because its walls contain par- 



Fig. 74. Ambulacral system of a starfish. 

 a, ampulla 1 ; a/-, Polian vesicles; c, circular 

 or ring canal ; m, madreporic plate or madre- 

 porite ; m' , madreporic or stone canal; /, 

 tube feet; r, radial canals; r' , branches to 

 ampulla?. (Alter Gegenbaur, from Parker 

 and Haswell's Manual.) 



