38 Dr. T. Williams on the Mechanism of Aquatic 



power is capable of being exerted on the strata of water inter- 

 posed between the leaves, under which it is driven forwards at a 

 speed regulated by the necessities of the breathing function. 

 The second force is the ciliary. The large cilia which occupy 

 the edges, and which are arranged in rows (fig. 3 a, a), are 

 capable of raising a sensible current. The minute cilia which 

 are distributed over the iiat surfaces of the laminse drive along 

 only microscopic streams. 



By means of a thin sharp scalpel, a section may be easily 

 made through the gill, parallel with the plane of the laminae, 

 and through the substance of the vault. If this section be made 

 about the middle of the gill, one of the largest laminae may be 

 readily detached and placed as an individual object under the 

 microscope. Such an object is represented in PI. V. fig. 3. 

 The dorsal margin curves like a reaping-hook (a, a). It is 

 stiffened by a large sickle- shaped cartilage {a, e) which extends 

 from the root to the apex : it supports the whole organ in situ. 

 Within this border, running along the side of the cartilage, 

 are also concealed two large vessels, with which many of the 

 ultimate blood-channels of the lamina are connected. Along this 

 dorsal edge are disposed two rows of large cilia, which propel 

 currents in two opposite directions, downwards along one side, 

 and upwards along the other. Every part of the leaf beyond 

 this cartilaginous border consists of a soft membranous sub- 

 stance (A), thickly vascular, composed, in fact, of little more 

 than vessels. The next feature to be noticed in the structure 

 of this branchial lamina is a series of duplicatures (c, c and b, b) 

 of this surface, and is singularly distinctive of the branchial 

 organ of this family of Mollusks. At first they look like acci- 

 dental folds of a delicate membrane ; but as they occur in every 

 single leaf throughout the organ, it follows that they are organic 

 formations. If the leaflet is laid carefully on the glass slip, 

 floating in water, and gently covered, without pressure y the true 

 character of these parts may be most clearly determined. 

 They consist undoubtedly of a bifid fold (B) of only one half 

 of the lamina, for a similar fold exists on the opposite surface. 

 Each fold is made up of two parts, which are united gradually 

 at either end, and separated by an interval in the middle. The 

 long axis of each fold is at right angles with the line of the 

 dorsal margin ; but the row is parallel with the latter. Slightly 

 beyond this row of plicae, and nearer to the centre of the leaflet, 

 is to be observed a second and smaller system [c, c). The folds 

 are formed in the latter case precisely as in the former. These 

 two systems of plications are separated from each other by a 

 narrow space of smooth membrane. All that portion of the 

 lamina which intervenes between the second row of folds and 

 the extreme apex [j) of the lamina is a plain unplicated surface. 



