TYPE ECHINODERMA. 581 



are generally present, the genus Cidaris and its allies alone 

 lacking them and possessing Stewart's organs. It is custom- 

 ary, therefore, to diyide the order into two sub-groups : the 

 Entobeanchiata, including the Gidaridce, and the EcTOBEANcm- 

 ATA, including all other forms, such as Strongylocentrotus and 

 Arhacia. 



2. Order Clypeastroidea. 



In this order, named from one of the genera contained in 

 it, the mouth still occupies the centre of the oral pole, but the 

 anus is situated in the interradius AB, either at the margin 

 of the flattened test, as in JEchinarachnius (Eig. 267), or on its 

 oral surface, as in Mellita. In 

 Clypeaster the body is but slightly 

 flattened, but in the two other 

 genera already mentioned the 

 flattening is carried to such an 

 extent that the test has a more 

 or less disklike shape, whence 

 the term Sand- dollars applied to 

 certain forms. 



In accordance with the shift- 

 ing of the anus from the centre „ „„„ . ^ t^ 

 ° . Fig 367 —A Cltpbastroid Echi- 



of the apical system certain jj^jj,, Echinarachnius parma. 

 changes take place in it, the prom the Aboral Surface 

 most marked being an extension with the Spines for the most 



» ,, T .J. J. I. 1 PART REMOVED (after A. Agassiz). 



of the madreporitorm tubercle 



until it includes all the genital and ocular plates, the genital 

 pores being forced outwards so as to lie on one of the inter- 

 ambulacral plates, and in some cases the pore of the posterior 

 interradius AB and the corresponding reproductive organ is 

 wanting (^Echinarachnius). The perforated ambulacra! plates 

 are as a rule confined to the apical portion of the aboral 

 surface, the plates near the margin of the test and those upon 

 the oral surface being imperforate. The perforated plates 

 are of course very narrow at the apical end of the series, and 

 gradually enlarge as they pass towards the edge of the test, 

 giving thus the appearance of five flower-petals, and hence 



