274 SYSTEMATIC ZOOLOGY. 



grouping which more accurate knowledge has shown to be 

 untenable. 



Chief among the features marking off the two groups are 

 the possession of a complete alimentary canal with mouth 

 and vent, and of a large body-cavity (ccelom) distinct from 

 the digestive tract. In a few cases, as in certain starfishes, 

 the vent may be small or entirely closed, but the fact that 

 in the larva the intestine opens to the exterior (Fig. 90, v) 

 shows that the condition of the adult is the result of degen- 

 eration. The large body-cavity or ccelom is distinct from 

 the other body-cavities in that it does not contain blood 

 and is without connection with the cavities of the digestive 

 tract and circulatory systems. 



The ambulacral system is also very characteristic. Its 

 structure is rather complex (fig. 91), consisting of (1) an 

 opening to the exterior either directly or by the inter- 

 vention of a perforated calcareous plate, the madreporite; * 

 (2) a stone-canal, connecting the madreporite with (3) a 

 circular or ring-canal around the mouth. This stone-canal 

 receives its name from the fact that its walls are usually 

 strengthened by deposits of lime. From the ring-canal 

 (4) a radial canal extends into each of the rays, giving 

 off at regular intervals pairs of canals which connect with 

 the (5) ambulacra. These last consist of two parts, the 

 ambulacra proper, which are on the outside, and the 

 am,pull(E, which project into the ccelom. Both these 

 structures are hollow and muscular, and the ambulacra 

 each terminate with a sucker. By contraction the fluid 

 of the ambulacral system is forced from the ampullae into 

 the ambulacra, thus extending them, while contraction of 



* In the adults of most Holotlmrians the madreporite opens 

 into the ccelom, but in the larvae of these the opening is to the 

 external world. 



