LAMELLIBRANCHIATA. 529 



the circumstances under which the tripartite heart of the Anodon is 

 developed, confirms that inference. In the Oyster the progress of 

 union of the two individuals has proceeded to the confluence of both 

 the two ventricles into one, and of the two auricles into one, and a 

 bipartite heart is the result : unless, indeed, in this and in other in- 

 equivalves, one individual or moiety of the divided and embryonal 

 mass be developed at the expense of the other, which may undergo 

 partial absorption. 



The ordinary course of confluence, by which, in the growth of the 

 13arenchyme from the line of union of the divided moieties of the 

 embryo, the two intestines become one, commences, as regards the 

 intestinal tube, from the anus ; and, being followed or accompanied 

 by the progressive co-approximation, co-adaptation, and ultimate 

 confluence of the two ventricles, these, through the relative position 

 of the rectum of each individual moiety, along the inner or mesial 

 side of the originally distinct ventricles, come to be included between 

 those co-adapting ventricles, and, ultimately, the single rectum becomes 

 included by the single ventricle, each resulting from the fusion of a 

 primitive pair ; and hence we are able to comprehend the cause of 

 that most singular anatomical fact which, hitherto, has been, without 

 any intelligible explanation, a mere empirical one, viz., the passage 

 of the rectum through the centre of the ventricle of the heart in most 

 Lamellibranchs. 



"We may discern in that well-marked and peculiar feature of the 

 Lamellibranchian development, viz., the almost complete fission of 

 the embryonal mass, with the establishment, in Anodon, of digestive 

 and circulating systems in each, an indication of the last remnant of 

 the parthenogenetic force, which, in its complete manifestation, ceases 

 with the Tunicata in the molluscous series. Nature, however, does 

 nothing abruptly, and a transitional character to higher kinds of 

 development of a single individual from a single ovum is indicated by 

 this attempt, as it appears to be, to form two individuals out of one 

 germ-mass. Their respective completion, however, as such, is checked ; 

 and development proceeds to form one (more complex) bivalve out of 

 two (less complex) univalve individuals. 



I may further remark, that the period and mode of the develop- 

 ment of the two valves of the incipient bivalve are subversive of 

 Oken's conjecture*, that the operculum of the operculated univalve 



* Defining the univalve moUusks, Oken says, " ^'on den beiden Muschel- 

 schalen wird eine rohrenforniig, umschliesst das ganze Thier allein, und die 

 andere bleibt als blosser Deckel iibrig, der die Kohrenmundung schliesst." — 

 Lehrbuch der Philosophie, Th. iii. p. 259. 8vo. 1811. 



" Of the two mussel (bivalve mollusks) shells " (which he treats of in the 

 preceding section), " one becomes excavated, tube-shaped, and alone incloses the 



M M 



