756 HOMOLOGIES AND AFFINITIES OF ECHINI. 



Cuvieria, and we thus have a marked antagonism between a dorsal and a 

 ventral surface. An anterior and a posterior extremity is of course definitely 

 fixed in all Holothurians, but a right and left only in those genera in which 

 the trivium forms a ventral surface in contrast to the less prominently de- 

 veloped biviutn. The same is the case in Echini. It is only in those genera 

 in which we find a bivium and a trivium that we have not only a right and a 

 left, luil an anterior and a posterior extremity. In the Galeritidae the 

 structure of the ambulacra and shape of the test alone would not be sul- 

 ficient to distinguish such a bivium and trivium as that of the Clypeastroids 

 and Spatangoids, and were it not for the position of the anal system in one 

 of the interambulacral spaces, we should have nothing to guide us in fixing 

 the anterior or posterior extremity (as in the Desmosticha ). much less a right 

 or left. By means, however, of the outline of the test, of the position of the 

 anal opening, ami of the structure of the ambulacra! petals, we can readily 

 lix the position of the longitudinal and transverse axis, of the anterior and 

 posterior extremity, and. of course, of right and left. In the Desmosticha the 

 animal rests upon an equal actinal portion of the five ambulacra! and inter- 

 ambulacral regions. In the Clypeastroids we find already a difference ; the 

 mouth is no longer in the centre, and the anterior part of the actinal surface 

 is frequently larger than the posterior | Kchinarachnius. Encope). In the 

 Petalosticha the actinostome is always anterior, so that the disproportion be- 

 tween the growth of the bivium and the trivium becomes very considerable 

 in such genera as Brissus, Metalia, and the greater number of the Spatan- 

 goids proper. We have therefore an actinal surface, the greater part of 

 which is formed by the posterior interambulacral area (the actinal plastron), 

 while of the trivium but a small part belongs to the actinal surface, and a 

 somewhat larger part of the posterior lateral ambulacra. So that what con- 

 stitutes the actinal surface in the Holothurians. Starfishes, Ophurians. Echini, 

 and Crinoids (Comatulae) is by no means Identical in the different suborders. 

 In Comatulae the actinal surface corresponds to the actinal surface of the 

 embryo starfish at the time it has just resorbed the pluteus, while the anal 

 opening is still placed on the side of the actinostome. The actinal surface 

 is identical in Starfishes and Ophurians, so far as the ambulacra! system is 

 concerned; while in Holothurians the actinal surface is analogous to the 

 actinal side of Clypeastroids when the actinostome is eccentric posteriorly. 

 and in which, supposing the mouth were marginal, we should have an actinal 

 surface formed of the trivium, and not of the bivium, as in Spatangoids. It 



