940 MOLLUSCA AND BRACHIOPODA 



as possess the requisite opportunity would apply themselves to the 

 study of the corresponding history in other Pectiiiibranchiate Gastro- 

 pods, with a view of determining how far the plan now described 

 prevails through the order. And now that these molluscs 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. 



Ciliary Motion on Gills. There is no object that is better 

 suited to exhibit the general phenomena of ciliary motion than a 

 portion of the gill of some bivalve mollusc. The Oyster will answer 

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

 gills of the Mussel (Mytilus), 1 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 ribbon- 

 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 disc. Or it will be 

 convenient to place the object in the aquatic box, which will enable 

 the observer to subject it to any degree of pressure that he may find 

 convenient. A magnifying power of about 120 diameters is amply 

 sufficient to afford a general view of this spectacle ; but a much 

 greater amplification is needed to bring into view the peculiar mode in 

 which the stroke of each cilium is made. Few spectacles are more 

 striking to the unprepared mind than the exhibition of such won- 

 derful activity as will then become apparent in a body which to all 

 ordinary observation is so inert. This activity serves a double pur- 

 pose ; for it not only drives a continual current of water over the 

 surface of the gills themselves, so as to effect the aeration of the 

 blood, but also directs a portion of this current to the mouth, so 

 as to supply the digestive apparatus with the aliment afforded by 

 the Diatomacece, Infusoria, &c. which it carries in with it. 



Organs of Sense of Molluscs. Some of the minuter and more 

 rudimentary forms of the special organs of sight, hearing, and touch 

 which the molluscous series presents are very interesting objects of 

 microscopic examination. Thus, just within the margin of each valve 

 of Pecten, we see (when we observe the animal in its living state 

 under water) a row of minute circular points of great brilliancy, each 

 surrounded by a dark ring ; these are the eyes with which this 

 creature is provided, and by which its peculiarly active movements 

 are directed. Each of them, when their structure is carefully exa- 

 mined, is found to be protected by a sclerotic coat with a transparent 



observations of M. Claparede on the development of Neritina fluviatilis (Midler's 

 Archiv, 1857, p. 109, and abstract in Ami. of Nat. Hist. ser. ii. vol. xx. 1857, p. 196) 

 showed the mode of development in that species to be the same in all essential par- 

 ticulars as that of Purpura. The subject has again been recently studied with great 

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



1 This shellfish may be obtained, not merely at the seaside, but likewise at the 

 shops of the fishmongers who supply the humbler classes, even in Midland towns. 



