414 Dr. T. Williams on the Mechanism of Aquatic 



than the innermost (e). By a poetical stretch of fancy it might 

 be said that, being composed of two sets of laminae, though 

 situated on the same side of the same base, they may justly be 

 likened to the oppositely-placed laminse of the plumose free gills 

 of the other Patelloid orders. But such a comparison would be 

 indeed far-sought. 



The branchiae of the vulgar Limpet, to the careless looker-on 

 on Nature's marvels, appear so contemptibly familiar, that to 

 subject such objects to a grave and minute philosophical exami- 

 nation must prove a severe trial to his patience and common 

 sense. Alas ! how short is the range of human sight ! Beneath 

 the familiar exterior of these common objects there lies an un- 

 discovered machinery of startling beauty and perfection ! These 

 little organs will serve to unite the branchial systems of the two 

 great groups of Mollusks, the Acephalous and Encephalous. 

 Through their aid it will prove practicable to establish a unity 

 of branchial type coextensive with the entire Molluscan series. 

 They will convince the zoologist that Nature never changes either 

 the plan of her action, or the design of her architecture, by 

 senseless and ludicrous transitions. Her incomparable, unequalled 

 skill lies in adapting a single principle to the most varied ends. 



Of the two rows of laminae in Patella as just stated, the inner 

 is composed of smaller leaflets than the more external. The 

 smaller and larger leaves occur alternately in the series. Both 

 are precisely the same in minute structure. Expert manipu- 

 lation is required in order to obtain a complete view of the gill- 

 processes in situ. The whole ring of the mantle to which they 

 are attached should be removed. Every portion of the loose 

 edge and fringe should then be cut away, leaving only so much 

 as will hold the laminae in position. A small segment of this 

 circle should be then placed in the cell of an object-glass, 

 floated with water and covered with a sHp of thin glass. The 

 position of the laminae may now be changed at will, by slackening 

 or extending the portion beyond the edge of the glass. 



It will now be remarked, that each lamina (fig. 8) is a separate 

 and distinct process, resting on, or proceeding from, a distinct 

 base ', that it is not a simple vertical sheet like the leaf of a book, 

 of equal thickness at every part, at the edge and at the base ; 

 that it is not, what seemed certain to the naked eye, a com- 

 pactly-structured single sheet. A far more intricate arrange- 

 ment discovers itself. First, each lamina, as it stands in its 

 place and unstretched, forms a concavo-convex outline (fig. 8). 

 It bulges out {a) like a sail in the wind on one side; it is hol- 

 low on the other (b) . This figure is due to the fact that the free 

 border (c) is denser and less extensile than the intermediate 

 membranous portion. Many advantages are secured by such an 



