DEVELOPMENT OF THE PECTINIBRANCHIATA. 337 



fifty a minute ; they are not always regular and frequently 

 change their character; strong pulsations occur among the 

 weaker pulsations. It frequently happens that the heart sud- 

 denly ceases to contract, remaining at rest for some time ; and 

 it is nowise uncommon to observe strong pulsations succeeding 

 intervals of rest. The cavity of the primitive heart is cylin- 

 drical, and is divided by a single partition. Its parietes are 

 excessively delicate, very transparent, and refract light in an 

 altogether different manner from the other organs (fig. Se) ; we 

 have never met with any liquid nor with any cellular structure 

 in them. We have also observed similar muscular tubes in 

 the two rounded lobes*; but here we see that many are super- 

 imposed and frequently ramify. This branching becomes more 

 and more abundant as we approach the periphery of the lobes, 

 where the finer ramifications frequently interlace and a muscular 

 network results (fig. 15 a, i) which serves to move the two rounded 

 lobes in all directions. It may be, that a vascular network 

 exists in some parts of this muscular reticulation, for little gra- 

 nules, refracting the light strongly, are to be met with dispersed 

 through the mass. Since the two distingiushed French natu- 

 ralists, MM. Milne-Edwards and Valenciennes, have pointed 

 out the greater or less incompleteness of the vascular system of 

 Mollusks, it has appeared to us to be important to observe the 

 condition of the circulation in the young Buccinum, but not- 

 withstanding all our efforts, we have not been sufficiently for- 

 tunate to detect the slightest trace of a circulating current f. 



We have already stated that the proboscis is one of the organs 

 that appears early ; it is recognizable by its cylindrical form and 



* Leydig, /. c, describes the structure of the muscles of a great number of 

 Gasteropoda, and remarks that the result of his researches upon this subject 

 does not altogether agree with that at which MM. Lebert and Robin have 

 arrived. We believe that those of Leydig are the more exact in all essential 

 points, inasmuch as they agree better with our own observations. 



f We cannot help remarking that there is something rery anomalous about 

 this supposed " heart," and we would venture to suggest that its structure and 

 position indicate rather, that it is no heart at all, but a *' contractile vesicle,** 

 similar to that which exists in the corresponding locality in the pulmonate 

 Gasteropoda, and which, acting in concert with the caudal vesicle, perfornvs 

 the functions of a heart before the true heart makes its appearance. See 

 Gegenbaur, Beitrdge zur Entwickelungsgeschichte der Land Gasteropoden^ 

 Siebold und Koiliker's Zeitschrift, 1852. [T. H. H.] 



SCIEN. UEU.—Nai. Hist. Vol. I. Part IV. 22 



