572 



INVERTEBRATE MORPHOLOGY. 



include all the plates of the apical system, and at the same 

 time the anal opening may leave its position near the centre 

 of the apical system and become situated in the iuterradius AB, 

 either at the margin of the flattened disklike test, or even on 

 its oral surface. A marked bilaterality of form is thus de- 

 veloped, which may become still more pronounced by a mi- 

 gration of the mouth away from the centre of the oral surface 

 along the line of the radius D, which at the same time be- 

 comes more or less altered in size and form, and consequently 

 dissimilar to the other radii (Fig. 263). In these cases it is 



possible to recognize in addi- 

 tion to oral and aboral surfaces 

 anterior and posterior poles 

 and a right and left side, the 

 median line of the body pass- 

 ing in front through the radius 

 D and posteriorly through the 

 iuterradius AB. Three of the 

 radii, C, D, and E, thus lie in 

 the anterior half of the body, 

 and for descriptive purposes 

 these have been termed the 

 trivium, while the two posterior 



ones, A and B. constitute the 

 FIG. 263 A PETALOSTICHOUS ECHI- . . 



KOID, Brinsopsis lyrifera, FROM ^iviwm. 



THE ABORAL SURFACE WITH THE The mouth, which is usual- 

 SPINES REMOVED (after A. AOASSIZ). ly situated in the centre of the 

 D = modified ambulacrum. y^j surfa is snrroTmde d by 



T ~ itisciolc 



an area, the peristome, which 



has imbedded in it only a few scattered calcareous plates and 

 consequently possesses a somewhat leathery consistency. 

 An oral system of plates cannot be distinguished in adult 

 Echinoids. 



The marked bilateral symmetry referred to above as occurring in cer- 

 tain Echinoids is undoubtedly a secondary condition, those forms in which 

 the mouth is central and the anus approximately so, and whose bilaterality 

 is indicated only by the madreporiform tubercle, being, there is every 

 reason to believe, the most primitive. The bilaterality cannot be regarded 

 as a reversion to the more primitive symmetry of the larva, since in the 



