INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 605 



minute examinations of diflferont forms, that in every instance tlio 

 aptyclii were almost entirely cellular, and lines of cell-tubes were 

 extracted from them, differing in scarcely any respect from the egg- 

 packets lying amidst the scattered eggs on the Ammonites serpentinus 

 of the upper lias. The facts he has collected are, he thinks, scarcely 

 consistent with the idea that the aptychus was simply an operculum, 

 but on the contrary tend to the conclusion that — possibly with the 

 siphuucular tube — it is an ovarian sac. 



Development of the Pulmonate Gasteropoda. — The conclusion 

 of M. Fol's essay on this subject, to the introductory portion of which 

 we have already directed attention,* deals with certain theoretical 

 points of some importance. 



Velum. — This larval structure, which is found in so many diflferent 

 animals, generally takes the form of an ectodermal thickening, covered 

 with vibratile cilia of a particularly large size ; it is generally circular 

 in shape, and is placed at the level of the mouth, and, as a rule, a 

 little above the buccal orifice. In most of the sea-dwelling MoUusca 

 it becomes very large ; thus, the superior extremity of the larva 

 becomes an enormous sinus, which is filled by the liquid of the body- 

 cavity, and is also provided with muscular fibres. On the other hand, 

 the Pulmonate Gasteropoda have the velum very small and even rudi- 

 mentary ; the thickening is not continuous and circular, but is only 

 developed at the sides ; it is, nevertheless, provided with branching 

 contractile cells, which are only found in the higher of the marine 

 forms ; so that, in these land-dwelling snails the velum is a structure 

 derived from and reduced from the more complicated forms. But the 

 process of reduction has not been uniform ; the " vibratile welt " has 

 undergone more diminution than have the sinus and the muscular 

 fibres, and that although these are not the essential parts of the velum. 



Hand in hand with this change in structure, it is evident that there 

 has been some change in function ; the primitive duty of the velum 

 was that of a locomotor organ, to which there was added on the 

 function of seizing nutriment ; in the Pulmonate forms this larval 

 structure has the function of circulating the nutrient fluid. 



The larval heart affords some difficulties ; in form and structure 

 it closely resembles that of Buccinmn and Purpura ; but it differs in 

 position, for they are primitively dorsal, whereas that of Helix only 

 gradually leaves a ventral position ; we require further information 

 before we can say whether the explanation of tliis difference is to bo 

 found in the fact that the jiist-mcntioned forms leave the shell at a 

 ' later period in development, or, whether they have their heart and 

 pallial cavity primitively placed on the ventral surface. 



By most authors the symmetry of the body has been ascribed to 

 the folding round of the shell ; Ihcring, however, regards the torsion 

 of the shell as due to the asymmetry of the viscera. ]\[. Fol regards 

 both these opinions as too extreme ; ho himself has already shown 

 that in the lietcropoda asymmetrical arrangements manifest them- 

 selves at an extremely early period. 



* Sec (intr, p. 414. 



