TYPE ECIIINODERMA. 535 



number of small plates between which the anal opening of 

 the digestive tract is to be found. Forming a ring around 

 this are frequently five plates possessing a radial position 

 which are termed the under-basals (Fig. 247, 2) and are un- 

 represented in certain forms ; next to these comes a second 

 circle of five plates, the basals (Fig. 247, 3), which are inter- 

 radial and correspond to the oral plates, while next to these 

 again is a third cycle, also of five plates, the radials (Fig. 247, 

 4), whose name denotes their position. Numerous other 

 plates may intervene in the various groups betAveen the radials 

 and the orals, but their number and arrangement is not suffi- 

 ciently constant to permit of homologies ; the oral and apical 

 systems are, however, represented more or less perfectly in 

 all but one of the classes, and consequently deserve special 

 mention. 



A well-developed dermal muscular system occurs in the 

 Holotlmrians in which the calcareous plates are scattered 

 and the body-wall consequently capable of considerable con- 

 traction and expansion, but in other forms it is very much 

 reduced. In those forms in which the calcareous plates are 

 simply in apposition strands of muscular tissue pass from 

 plate to plate, a considerable amount of movement being pos- 

 sible, but in the Sea-urchins, for example, the dermal muscu- 

 lature is almost wanting, being reduced to bands passing to 

 the bases of the movable spines and to the complicated mas- 

 ticatory apparatus. 



The ccelom is somewhat complicated in its relations, which 

 vary cousiderabl}' in the different groups. In all enteroccelic 

 and schizocoelic portions are distinguishable, the former in 

 the embiyo arising as pouchlike diverticula from the primi- 

 tive intestine or enterou, and later becoming completely con- 

 stricted off from it. Much variation occurs in the later his- 

 tory of the pouches in the various groups, but in general it 

 may be stated that one of them, the left, has a portion con- 

 stricted off from it, which forms the adult water vascular sys- 

 tem or hydroccel, a structure characteristic of the Echiuoderms ; 

 and furthermore this same left enteroccel communicates with 

 the exterior by a dorsal pore, situated in the iuterradius CD, 

 and represented in the adult by one or many pores opening 



