434 Prof. S. Lov^n on the Structure of the Echinoidea. 



This is the diversity that the right interraclium 1 shows in 

 all known genera of recent Spatangida3. It is quite clear that 

 it is not the left side that has a plate more, but the right that 

 has a plate less, and that here within a region not far from the 

 peristome, but separated therefrom, the normal structure of the 

 plates is disturbed. It is in the direction of this interradium 

 that the diameter lies in relation to which the ambulacra are 

 symmetrically disposed. Whether this diameter may possibly 

 indicate the heterologous position which the Echinoderm, whilst 

 still residing in its larva, had with regard to the latter, a pri- 

 mordial axis, from which it subsequently passed to another 

 antero-posterior axis, or whether this abnormal coalescence 

 of certain plates on one side of the animal may perhaps have 

 some connexion with what takes place when the Echinoderm 

 took up into itself the stomach and intestine of its larva, are 

 questions which deserve to be borne in mind during further in- 

 vestigations upon the development of the Echinodermata, but 

 which, for the present, we can only treat in a general way. 



The unpaired interradium differs far more from the paired 

 in the Spatangidas than in the other irregular Echinoidea. It 

 has a far more independent structure ; and its dissimilar 

 plates, which are essentially symmetrical, are differentiated for 

 various functions. The first, or peristomial plate, which in the 

 young is not very unlike those of the paired interradia, becomes 

 developed into the labrum, with which the animal, during its 

 movement forward in the soft sea-bottom, raises the mass of 

 mud which constantly fills its intestinal canal. Behind the 

 labrum follows the sternum, composed, like all the following seg- 

 ments, of two paired plates, furnished with powerfully movable, 

 more or less oar-shaped radioles, with which the animal rows 

 itself away ; and behind the sternum the episternum and the 

 long row of the abdominal plates, which are generally numerous, 

 and which, in most of the existing genera, close at the ma- 

 dreporite, or, in Hemiaster and many extinct genera, are sepa- 

 rated therefrom by eye-plates or vertical plates, when these 

 close together behind the madreporite. 



The labrum, in most, is very short, so that, with its 

 outer margins, it occupies only the first 2-pored radial plate ; 

 in others, such as Maretta, Lovenia, Breyyiia, EupataguSj 

 Atrapus, and Pakeostoma, it is so produced backward that it 

 corresponds to the two or three first radial plates. The sternum 

 presents, most distinctly in the Prymnodesmii, a certain corre- 

 spondence with the plates 2 of the paired interradia, inasmuch 

 as it is usually small when these are very large, as in Breynia^ 

 Plagionotusj Eupatagus^ and Lovenia. Its relation to the biviura 

 has already been mentioned. The plates of the episternum, in 



