20 ECHINODERMA GENERAL DESCRIPTION 



Half-way between the perradial planes are the planes marking the 

 interradii. Organs bisected by these are " interradial " ; such are 

 the interambulacral areas of the test, the oral plates of Crinoidea, 

 the gonads of Echinoidea. Between the perradii and interradii 

 are adradii, a term little used in practice ; thecal plates adjoin- 

 ing the ambulacrals are called " adambulacral." In a regular 

 pentamerous Echinoderm an interradius is opposite to a perradius, 

 and an adradius opposite to an adradius. 



All Echinoderms have a bilateral symmetry. Primitively the 

 plane of symmetry, the sagittal plane, is determined by the mouth 

 (anterior), the anus (posterior), and the hydropore (dorsal). But, 

 in the first place, this sagittal plane, when clearly shown, is not 

 the same as the sagittal plane of the Dipleurula. In Pelmatozoa 

 it certainly is not ; in Holothurians it only approximates to it. 

 Secondly, in Echinoidea and some Cystidea, and in such Asteroidea 

 as have an anus, the plane passing through the vertical axis and 

 the madreporite (M plane) is not the same as that passing through 

 the vertical axis and the anus (anal plane). Thirdly, the rela- 

 tions of the anal plane to the M plane and to the radii may vary 

 even within a single class, e.g. Echinoidea and Cystidea. Conse- 

 quently the selection of any one plane as a plane of orientation 

 for the different classes is arbitrary. Also it is convenient. We 

 take then the M plane and note that the hydropore lies in an in- 

 terradius with a radius opposite to it (Fig. XVIII.). That radius we 

 denote by A. Then placing the animal with its mouth upwards 

 and going round the test in the direction of the watch-hand (i.e. 

 dextrally), we denote the other radii in order, E, C, D, E. The 

 hydropore lies in interradius CD. In a developing Holothurian, 

 or in such Holothurians as retain an external madreporite (Fig. 

 XVIII. 3), the anus and mouth both lie in the M plane, forming 

 the poles of the long axis, while radius A bisects the ventral 

 surface ; this therefore is the sagittal plane of bilateral symmetry, 

 and Cu6not, 1891, calls it the " Holothurian plane." In a Crinoid 

 (Fig. XVIII. 1), anus, mouth, and aboral pole, all lie in the M 

 plane, which here also is the sagittal plane ; but the anus, in inter- 

 radius CD, never marks the aboral pole of the main axis, though 

 it may usurp the place of the mouth at the upper pole. Many 

 Cystids, and apparently the Blastoids, have a similar orientation. 

 Other Cystids differ in that the anus lies to left or to right of 

 the hydropore, while the relation of the radii to the M plane 

 is not clearly defined. In Echinocystis (p. 301), which probably 

 represents the relations in the primitive Echinoid, the symmetry 

 remains as in Pelmatozoa ; while the mouth is at one pole of the 

 main axis, the anus lies in or near the M plane, which is therefore 

 the sagittal plane, but the madreporite is near to the aboral pole. 

 In later Echinoids the case is altered (Fig. XVIII. 4) ; the first step 



