316 ON THE CLASSES OF THE 



refer those who may wish to be acquainted with them to 

 the perusal of his description. 



The Medusa bring us to the genera Porpita, Felella, 

 &c. of M. Lamarck, which compose his Radiaires ano- 

 males, and which may for greater convenience be desig- 

 nated, as they have already been by M. Cuvier, under their 

 ancient name of Acalephdi. The genera Lucernaria and 

 Actinia lead us from these animals to the Fistidida of 

 Lamarck. And as M. Cuvier has pointed out the afiBnity 

 which some Holothuricc bear to the Echinides of Lamarck, 

 we may conclude that the chain of radiated animals returns 

 into itself, after presenting the following five principal 

 constructions^ 



echinida, 



Stellerida, 



Medusida, 



Acalephida, 



Fistulida. 

 These classes form a group of animals distinguished 

 by the disposition of their parts, internal as well as exter- 

 nal, to radiate as from a centre. The Polypes indeed 

 among the Acrita have shown vestiges of this disposi- 

 tion; but nowhere is Radiation a regular, nowhere is it a 

 constituent part of the formation, except among the ani- 

 mals with the consideration of which we are at present en- 

 gaged. The Radiafa besides are perfectly distinguished 

 from the Acrita by the possession of a system of respira^ 

 tion and apparently of a definite nervous system ; but 

 this as yet has only been detected among a few. Asterias 

 rubens is almost the only species of the Radiata of which 

 the anatomy may be said to have been well investigated. 

 Dr. Spix, to whom the honour of this investigation is 

 due, apjwars to consider the sexes to be distinct. His 



