L. Agassiz on the Echinodermata. 37 



Echinodermata. I shall therefore merely remark, in general, 

 that the teeth alternate with the ambulacral spaces and are 

 placed directly opposite to the middle of the interambulacral 

 spaces, between the two series of plates which compose these, 

 and to which they are attached by means of a very compli- 

 cated apparatus. The structure of this apparatus, which is dif- 

 ferent in different genera, I shall describe as soon as my re- 

 searches respecting it are more complete. As the teeth corre- 

 spond to the interambulacral spaces, it is obvious enough that 

 there must be an odd one. It is this odd tooth that is found 

 behind on the antero-posterior axis itself. Its motion is di- 

 rected forwards from the hinder part. The four other teeth 

 are in pairs, and move laterally from right to left and from left 

 to right in contrary directions on both sides of the mouth. 

 Their motion may therefore be compared with that of the max- 

 illae of articulated animals, while the odd tooth may be consi- 

 dered as bearing some analogy to the lower lip of some of 

 these. The space between the teeth and the lower orifice of 

 the testa is covered with small moveable plates similar to those 

 which surround the anus. 



It is scarcely necessary to observe that the determination of 

 these points must be of the utmost importance in examining 

 the situation of the intestines and their mutual relations. Un- 

 fortunately, the state of the specimens which I had intended 

 for dissection, did not allow me to investigate thoroughly all 

 those points which I had wished to decide ; and when I was 

 dissecting some fresh specimens at the sea-side, I was not yet 

 aware that it was possible to determine so precisely and posi- 

 tively the relation of the different regions of the body of these 

 animals. 1 am therefore now obliged to postpone all further 

 detail until I have completed this part of my labour. The 

 facts which I have just stated appeared to me sufficiently re- 

 markable to warrant their separate publication without an im- 

 mediate review of the whole organization of this class. As to 

 the manner in which I have designated the several regions of 

 the Echinodermata, it may be objected that, most of these 

 animals having the mouth beneath the disc, the side on which 

 it is found cannot be considered as the anterior region of the 

 body. But this denomination is no less correct on that ac- 



