VIII 



ECHINODEBMATA— MORPHOLOGY OF SKELETON 



319 



A. The Apical System (Calyx). 



I. Echinoidea. 



The part of the test which in the Sea-urchins is formed by the apical 

 system varies greatly in size. In the older and more primitive forms, 

 the regular Echinoidea, it is still somewhat extensive as compared with 

 the rest of the test (Fig. 271), but in modern, especially irregular forms 

 (Clypeadridw and Spantangidm), it continually diminishes in relative 

 size till it is nothing more than a minute region at the apical pole. It 

 is possible to deduce the apical system of the Echinoidea directly from 

 the hypothetical primitive form by the help of certain Saleniidce (Fig. 

 :272). It is true that in the apical system of this family, as in that 

 z 



Fig. 271.— Tiareohlnusprlnoeps, Laube (after LovSn). 

 1, Genital aperture ; 2, anus ; 3, basal ; 4, radial ; 5, 

 ambulacrum ; 6, the three upper plates of an inter- 

 ambulacrum. 



of all other Echinoidea, the infrabasals are entirely wanting, but all 

 the other typical plates are present : i.e. a central plate, and round it 

 five basals, and outside these, alternating with them, five radials. In 

 the right posterior interradius each of the three plates, the central 

 and the two basals, is incomplete at the point where they meet. A 

 circular region, the anal region, in which the anus lies, is thus formed. 

 The anus, therefore, here lies asymmetrically in the apical system, and 

 this' is the case in most Falceechinoidea and in most regular Euechinoidea. 

 According to the universally accepted terminology, it lies in the right 

 posterior interradius. 



The typical system above described for the adult Saleniidce has 

 been found to be repeated in very young specimens of other Sea- 

 urchins examined for this purpose (Echinus, Fig. 273 ; Toxopneustes, 

 Fig. 274). 



Apart from these cases, where a primitive condition is shown by 



