42 L. Agassiz on the Echinodermata. 



that stand nearest to each other. We should be mistaken how- 

 ever as to the growth of the Echinodermata, did we think that 

 there is a generic connexion between the plates, on account 

 of their forming vertical series from the mouth to the summit of 

 the disc. It has been already remarked that the plates of each 

 space are alternately a little more elevated than each other, but 

 no attention has been paid to the manner in which the plates 

 of all the spaces succeed each other in the same Echinus ; and 

 yet, if we consider it closely, we shall see that the new plates 

 are developed in spiral lines, passing, without interruption, 

 from one series to another through all the spaces from the 

 circumference of the mouth to the dorsal summit, so that those 

 which rest on each other in a vertical line do not make their 

 appearance in immediate succession. It appears to me well 

 worthy of remark, that in these animals, holding so low a rank 

 among organized beings, we should find the succession of the 

 solid parts composing their integument so strikingly analogous 

 to the arrangement of the leaves around the stems of plants ; — 

 an arrangement, the laws of which have been recently disco- 

 vered by M. Schimper, and explained, so far as regards the Co- 

 niferce, in a memoir of M. Braun on the arrangement of the 

 scales of their cones. 



The small plates surrounding the mouth and those around 

 the anus are arranged in a peculiar manner ; they are easily 

 moved, and thus facilitate the deglutition of the food and the 

 voiding of the excrements. In general the testa of the Echini 

 is not so immoveable as one who had not observed them in a 

 fresh state might be led to suppose. All the plates forming the 

 upper part of the disc are often set in motion : sometimes they 

 sink, sometimes they rise, and, in the oblong species, the lon- 

 gitudinal diameter is often extended beyond its ordinary length. 

 The great mobility of the spines, the variety of their move- 

 ments, and the manner in which they help the animal to seize 

 its food, have been already noticed. 



The growth of the starfishes and the Crino'ides will appear 

 to be carried on by a process exactly the same, as soon as H is 

 agreed that an ambulacral space of an Echinus answers to an 

 ambulacral surface of a starfish, and that an interambulacral 

 space of the former answers to the large marginal pieces of 



