29f> 



Prof. Agassiz on the Echinodermata. 



wards beings more and more complicated, arrive at the very summit 



of the series, viz. Amanita casarca. 



The various developments which the unfolding of this new subject 

 for contemplation would allow of, would carry me beyond my object. 

 Far from having exhausted, I have, alas ! scarcely glanced over it. I 

 leave this to those who are more capable. I could attempt only a 

 feeble sketch ; may it at least be sufficient to guide the reader in the 

 midst of the numerous difficulties which await him in the study of 

 this great and important class of the vegetable kingdom. 



Paris, Feb. 1, 1841. 



XXXIV. — Observations on the Progress recently made in the 

 Natural History of the Echinodermata. By Prof. Agassiz. 



[Continued from p. 197.] 



In my c Prodromus' I expressed doubts as to the membra- 

 nous ambulacral tubes in the Sea-urchins having any rela- 

 tion to their powers of locomotion, grounding my opinion upon 

 some observations which I had made on the sandy shores of 

 Normandy, and upon the very positive assertion of Aristotle, 

 who tells us that they move by the aid of their spines ; and 

 that even by the state of these organs their degree of pro- 

 gressive power may be known (liv. iv. chap. v.). Mr. Forbes, 

 however, has shown this view of the matter to be erroneous, 

 and has demonstrated that they also progress by means of 

 their ambulacral tubes, especially when upon solid surfaces. 

 In company with this gentleman I have seen them ascend, 

 by the aid of these tubes, the perpendicular sides of a smooth 

 glass vessel. No further evidence could be wanted to set at 

 rest the point in question. It yet remains to be shown whe- 

 ther, among the Cidarites, the long club-shaped spines are 

 not the principal organs of motion, and that it is among the 

 Sea-urchins with very short bristles that the ambulacral tubes 

 are essential to this function. M. de Siebold mentions the 

 existence of microscopic cilia in the interior of these tentacula 

 and of their vesicles (Mill. Archiv, 1836, p. 295). M. Ehren- 

 berg, on the other hand, describes the vibratory movements 

 in the membrane of the spines of Echinus saxatilis (move- 

 ments which are denied by Mr. Forbes), and indicates the 

 existence of an internal circulation of corpuscles, similar to 

 the globules of blood, in the retractile tentacula upon the dor- 

 sal face of the Asterias violacea ; he adds, moreover, that the 

 surface of these tentacula is entirely covered with vibratile 

 cilia (Miil. Archiv, 1834, p. 577). M. Volkmann has also 

 given some new details upon the circulation in the Asterice, 



