MOLLUSCOUS ANIMALS GENERALLY. 189 



creature, consisting of two large ciliated lobes with scarcely the rudiment 

 of a body, may be seen in active motion among them. This may happen, 

 indeed, not only to one but to several embryoes within the same capsule, 

 especially if their number should be considerable; for it sometimes appears 

 as if there were not food enough for all, so that whilst some attain their 

 full dimensions and complete development, others remain of unusually 

 small size, without being deficient in any of their organs, and others 

 again are more or less completely abortive, the supply of supplemental 

 yolk which they have obtained having been too small for the develop- 

 ment of their viscera, although it may have afforded what was needed for 

 that of the ciliated lobes, eyes, tentacles, auditory vesicles, and even the 

 foot, or, on the other hand, no additional supply whatever having been 

 acquired by them, so that their development ha?s been arrested at a still 

 earlier stage. These phenomena are of so remarkable a character, that 

 they furnish an abundant source of interest to any Microscopist who may 

 happen to be spending the months of August and September in a locality 

 in which the Purpura abounds; since, by opening a sufficient number of 

 capsules, no difficulty need be experienced in arriving at all the facts which 

 have been noticed in this brief summary. 1 It is much to be desired that 

 such Microscopists as possess the requisite opportunity, would apply 

 themselves to the study of the corresponding history in other Pectini- 

 branchiate Gasteropods, with a view of determining how far the plan now 

 described prevails through the Order. And now that these Mollusks 

 have been brought not only to live, but to breed, in artificial aquaria, it 

 may be anticipated that a great addition to our knowledge of this part of 

 their life-history will ere long be made. 



586. Ciliary Motions on Gills. There is no object that is better 

 suited to exhibit the general phenomena of Ciliary motion ( 4o5), than 

 a portion of the gill of some bivalve Mollusk. The Oyster will answer 

 the purpose sufficiently well; but the cilia are much larger on the gills of 

 the Mussel,* as they are also on those of the Anodon or common ' fresh- 

 water mussel' of our ponds and streams. Nothing more is necessary than 

 to detach a small portion of one of the riband-like bands, which will be 

 seen running parallel with the edge of each of the valves when the shell 

 is opened; and to place this, with a little of the liquor contained within 

 the shell, upon a slip of glass, taking care to spread it out sufficiently 

 with needles to separate the bars of which it is composed, since it is on 

 the edges of these, and round their knobbed extremities, that the ciliary 

 movement presents itself, and then covering it with a thin-glass disk. 

 Or it will be convenient to place the object in the Aquatic-box ( 122), 

 which will enable the observer to subject it to any degree of pressure that 



1 Fuller details on this subject will be found in the Author's account of his 

 researches, in "Transactions of the Microscopical Society," 2d Ser., Vol. iii. 

 (1855), p. 17. His account of the process was called in question by MM. Koren 

 anjd Danielssen, who had previously given an entirely different version of it, but 

 was fully confirmed by the observations of Dr. Dyster; see "Ann. of Nat. 

 Hist." 2d Ser., Vol. xx. (1857), p. 16. The independent observations of M. Clapa- 

 rede on the development of Neritina fluviatilis (Mullens "Archiv," 1857, p. 109, 

 and abstract in "Ann. of Nat.*Hist.," 2d Ser., Vol. xx. (1857), p. 196, showed the 

 mode of development in that species to be the same in all essential particulars as 

 that of Purpura. The subject has again been recently studied with great minute- 

 ness by Selenka, " Niederlandisches Archiv fur Zoologie," Bd. i., July, 1862 



2 This Shell-fish may be obtained, not merely at the sea-side, but likewise at 

 the shops of the fishmongers who supply the humbler classes, even in midland 

 towns. 



