vui EGHINODERMATA— MORPHOLOGY OF ^KELF.TON 317 



While these terms facilitate the mopphologieal description of the 

 body they do not take into account its position in the water, or 



with regard to the sea-floor, which is assumed to be liorizontal. 

 Thus the normal position of the Star-fish and Sea-urchin is such that 

 the oral zone is directed downwards and the apical zone upwards ; 

 while the very reverse is the case in the Crinoids, where the oral 

 zone faces upwards and the body is attached to the substratum by a 

 stem which is inserted at the apical pole. In the Holothurians, again, 

 the principal axis of the body lies parallel to the substratum, and the 

 oral pole forms its anterior, the apical pole its posterior end. 



For particulars as to the form of the body and the external 

 organisation of the various classes and orders of the Echinodermata, 

 cf. the Systematic Eeview, and also specially the two sections which 

 treat of the skeletal and ambulacral systems. 



11. Morphology of the Skeletal System. 

 Meaning of the Most Important Lettering of the Figures. 



(In many of the diagrams of the apical system of various Echiuoderms tlie 

 infrabasals are dotted, the basals shaded with concentric lines, and the radials 

 marked black. The brachials of the Crinoids are shaded with radial lines.) 



Introduction. 



The extensive comparative and ontogenetic researches which have 

 been made on the Echinoderms have shown that it is to some degree 



