Mr. Robert Garner's Malacological Notes. 359 



forms at all, or contemporaneously, or closely following it, the 

 molluscous shell, foot, and ciliated velum may appear, either 

 in the above forms of development or in another, where upon 

 the segmented morula a germ-lamella makes its appearance, 

 in which the formative cleavage process is very active, so that 

 the lamella grows quickly and surrounds the other residuary 

 part of the yelk, which in some cases, as in Sepia, but not in 

 the Argonaut, constitutes a large fund of sustenance for the 

 young embryo*. As we have above observed, many of these 

 and other modifications cannot, with our present knowledge, 

 be reduced to a consistent system, nor can the succeeding stages 

 always be so. Thus, for ourselves, an ascidian is manifestly 

 allied to a bivalve mollusk, as much as the latter is to a gas- 

 tropod, and Ave hold that morphology suffices to prove this ; 

 yet, on the other hand, very different are the early forms of 

 the two, though in this case appropriate enough — one qualified 

 for preliminary locomotion, the other, it may be, to immedi- 

 ately fix itself ; indeed the former larva is so tadpole-like that 

 it has been considered by Schmidt and others, as is well 

 known, to foreshadow the vertebrate animal, though it is 

 probably truly included in the molluscous community, its ap- 

 pendage comparable, it may be, to the tail of the Carinaria, to 

 the Jlagellum of some bivalves, or even to the foot of the deve- 

 loped animalf. The veligerous stage, pretty general in, though 

 not peculiar to, gastropods, seems a form well adapted for their 

 dispersion, and indeed, in certain species, may perhaps be said 

 to continue through life. Here the embryo is furnished with 

 a ciliated disk or velum, and an external spiral shell is formed 

 with an operculum attached to the foot, a shell even existing 

 in the larva of the ultimately naked species ; but in such naked 

 species as have an internal shell (the Pulmonata having also 

 other peculiar modifications in their development \) it is intra- 

 dermal. With respect to the Cephalopoda, Owen says, 

 " were growth superinduced at any arrested stage of cephalo- 

 podic development, no known inferior order of mollusk would 

 result." Witli oneness of the animal plan in general, purpose 

 is only accomplished by much variation and adaptation, as well 

 as suppression or development of the homologous organs, even 



* " Developmental History of Mollusca," by Prof. E. Ray Lankester, 

 Phil. Trans, vol. cLxv. There is a clear and succinct account of the em- 

 biyology of the Gastropod by M. A. Giard, Ann. & Mag. Nat. Hist., April 

 1876. 



t Mr. Huxley compares the posterior prolongation of Carinaria to that 

 of Strombus, considering it to represent the metapodiiitn. 



\ These peculiarities are observable in the egg^ of Helix aspersa, easily 

 procurable, or better in the West-Indian Bulimi. See also Woodward's 

 'Manual,' p. 287. 



25* 



