528 LECTURE XXI. 



pearance of the bed of living individuals is due to their dying out 

 through influences, which Prof. Forbes himself recognises and ex- 

 plains, as unfitting the ground for further sustenance and increase of 

 the tribe ; and that the appearance of a colony in another position is 

 due to the progressive increase from a few individuals, and not to the 

 migration of the whole colony. 



So far as observations have been made on the development of 

 bivalves, although it is true that, as yet, they have been, with 

 few exceptions, limited to the fresh-water kinds, the young Lamelli- 

 branch, however free the parent, is attached at the beginning 

 by a byssiforra peduncle : it has no lamelliform gills, but respires by 

 the vascular and ciliated surface of the mantle-lobes, and it has two 

 distinct hearts. 



In all these characters we perceive the manifestation, though it be 

 transitory, of some peculiarities of the Brachiopodous order ; and 

 hence we derive an additional argument for concluding that the Bra- 

 chiopods are on a lower step of molluscous organisation than the 

 Lamellibranchs : and this is congruous also with the earlier appear- 

 ance and multiplication of the Brachiopods in the primeval seas of 

 this planet. 



The contemplation of the phenomena of the development of 

 Anodon also makes us comprehend better the cause of some of the 

 peculiarities of the structure of the Lamellibranchiate mollusks. 

 There seems to be in that development a tendency, at the begin- 

 ning, to form two individuals out of one germ-mass, — to develope 

 two univalves in the place of one bivalve : and some of the differences 

 of -structure characteristic of different bivalves evidently relate to 

 the arrest of development at given stages of the progressive confluence 

 by which the two primary individuals are ultimately fused into one. 

 Thus we may regard the two separate hearts of the Brachiopods as 

 a retention of that distinctness or doubleness of the organ which it 

 manifests at its first appearance in the embryo Lamellibranch, when 

 not only the auricles, but the ventricles, are on separate halves of the 

 almost divided body. In Area, the ventricles have approximated 

 but not coalesced. In Anodon the two ventricles have coalesced, 

 but not the two auricles, wliich continue permanently separate, and 

 impart the seemingly complex condition of the reptilian tripartite 

 heart to this inferior form of mollusk. It has been long shown, how- 

 ever*, how the function of the two auricles of the bivalve being the 

 same, that very doubleness or repetition of the part became a mark 

 rather of inferiority, as compared with the more compact form of the 

 single-auricled heart of the univalve mollusk ; and we now see that 



* X. vol. ii. p. 33. 



