220 ON THE CLASSES OF THE 



among themselves some very beautiful and extraordinary 

 types of form, than which none however is more interest-? 

 ing than the composite sheathed Polype. But this exter- 

 nal stony sheath disappears in the Sponges and Alcyonia, 

 or at least confounds itself with the common body of the 

 Polypes. These very imperfect beings are supposed to con- 

 nect together the Polypi vaginati and the Polypi natantes, 

 that is, the two most complex forms of the circle ofAcrita; 

 and we thus, for the first time, see that in passing from 

 one perfect plan to another, Nature makes use of some of 

 her very simplest constructions. By means of the Sponges 

 and the Polypi tubiferi of Savigny and Lamarck, we arrive 

 at the Polypi natantes, where the structure of a sheathed 

 Polype is completely reversed. It is the axis of the whole 

 compound animal which is here stony, often hollow, and 

 about which the Polypes combine to form a fleshy body 

 of a constant and regular form. M. Savigny, whose dis-» 

 coveries in every branch of Natural History he endeavours 

 to elucidate are equally important and interesting, considers 

 the internal organization of the Polypi tubiferi, which 

 he finds so complicated in comparison with the animals we 

 have as yet had under consideration, to be analogous to that 

 of the genus Veretellum among the Polypi natantes; and 

 indeed the axis of these last is no longer distinct in the 

 Veretellum Cynomorium (Pennatula Cynomorium Pall. J. 

 The compound structure of the Polypi natantes however 

 is not so very observable in the genus Virgularia, which 

 has neither the general form nor the habits of the rest. The. 

 animals composing this genus present a linear filiform body, 

 which is sunk in the sand or mud so as to leave nothing but 

 the polypiferous extremity of the animal exposed. Having 

 thus clearly receded from the type of the Polypi natantes.^ 



