Prof. J. Miiller on the Structure of the Echinoderms. 7 



and a biradial segment, a tritium and a bivium. In the Sea- 

 urchins of the genus Dysaster, trivium and bivium are actually 

 separated by a wide interspace ; but in most quinquepartite 

 Echinoderms they may be discovered and the position of the 

 radii thence determined. Between the two radii of the bivium 

 lies the anus, or it moves away from the mouth to the apical 

 pole, and even beyond it, to the opposite azygos radius. 



If the azj^gos radius be unknown, it may be determined by 

 the meridian of the anus, which in Cidaris and Echinus is ap- 

 proximated to one of the tive ambulacra. If the azygos radius 

 be known, but the anus central, as in the HolothuriadcB with am- 

 bulatory regions, the meridian of the anus may be determined 

 from the azygos radius of the elongated form. 



We may lay down then, upon the ideal spherical Kchinoderm, 

 the oral and apical pole; a triradial segment and a biradial seg- 

 ment, with the anal area. If the ambulacral arese of this Echi- 

 noderm are equally developed in all directions, it will become an 

 Echinus with its mouth directed downwards. The sphere is as 

 it were in equilibrium in this position ; it is, however, traversed 

 by an ideal plane which passes through the meridian of the 

 azygos radius. If it be rotated, within this plane, out of its 

 position of equilibrium, in such a manner that the bivium occu- 

 pies more of the ventral surface than the trivium, we have the 

 position of the Spatangida. If within the plane, the globe ro- 

 tate in the opposite direction, so that the trivium alone occupies 

 the ventral surface, the result is the position of the Holothuriada. 

 In this ideal globular model of the quinquepartite Echinoderm 

 we have then the following points fixed, and good for all pos- 

 sible cases, viz. the oral and apical poles and their axis ; the con- 

 trasted tri\-ium and bivium ; the longitudinal plane passing 

 through the meridian of both, and dividing the Echinoderm 

 into two equal parts ; and finally, the anal area. In the creeping 

 Echinoderms the oral pole may be directed downwards or for- 

 wards, the apical pole upwards or backwards ; the trivium, with 

 its azygos radius, may sometimes be presented forwards, some- 

 times downwards ; the bivium, with its azygos inter-radius, 

 sometimes backwards, sometimes upwards ; the anus doNNTiwards 

 and backwards, upwards and backwards, or upwards. 



The radial segments of the Echinoderm have an oral pole, and 

 opposed to this an apical pole. The mouth, surrounded by the 

 circular canal of the ambulacra, lies in general in the centre of 

 the oral region, rarely on one side of it. The surface between 

 the poles may be divided into ambulacral and inter ambulacral 

 segments. By the extension of the former in breadth, the latter 

 may entirely disappear, as in the Holothurice sporadipodes. In 



