262 On the Mechanism of Aquatic Respiration. 



lamellar water-tubes ; q, q, q, are the superior lobes of the single 

 loop, of which the free margin of the gill in Mytilus is composed ; 

 p, is the lower lobe. The deep groove between these lobes is 

 the great alimentary water-channel, bearing a current moving in 

 the direction of the mouth. I, is a fleshy nodule, by which the 

 loops are united into a series ; k, k, k, are horizontal water- 

 passages ; m, m, fleshy nodules, which connect the contiguous 

 bars. The system of arrows, g, g, g, indicate the direction of the 

 great respiratory current, along the upper surface of the lamella ; 

 i, i, i, the lower : both having the same direction — towards the 

 free margin of the gill. 



Fig. 18. Is a transverse section of one tube from the gill of Mytilus, 

 exhibiting the mode in which the lamina? separate, in order to 

 form a tube (a, b). 



Fig. 19. Four bars from the same gill, illustrating further the vertical 

 parallel planes on which the penknife-shaped bars are placed, and 

 the mode more exactly in which they are tied together. 



Fig. 20. A magnified view of a minute portion of a single blade-like bar 

 from the gill of Mytilus. It illustrates the distribution of the 

 cilia; and the water-currents, denoted by the arrows, set in 

 motion by them, a, b, are intended to show the position of the 

 blood-channels in the axes of the thickened lines of either edge 

 of the blade. If the upper lamella in the exceptional gills of 

 Mytilus should be hereafter proved to carry only a single system 

 (afferent or efferent) of blood-currents, the channel carrying such 

 a single current must prove to be aflat passage, whose transverse 

 section would extend from a to b ; e, e, fy c, fleshy nodules. 



Fig. 21. Two longitudinal lobes, or pectinations, from the gill of the 

 common Oyster, a, c, the double loops of which the free margin 

 is composed. They are drawn as if separated from each other, 

 in a tubular form ; but, naturally, the two planes of loops lie in 

 close apposition. At this border in Ostrea, the branchial bars are 

 soldered together by a continuous membrane over the interval 

 included in the dotted lines g. f, are the transverse pieces of the 

 intra-tubular framework. By these transverse pieces, the lamellae 

 forming the water-tubes are sometimes drawn into quadrilateral 

 figures, d, e. 



Fig. 22. A few of the "bars" from the same gill, showing that each bar is 

 an independent vessel. The component hyaline cartilages, d, c, 

 are brought close together, so as to form a cylindrical channel, e. 

 a, b, exhibit the transverse structures, as running along one (the 

 internal) of the tubes. This arrangement proves that the latter 

 cannot be transverse vessels. 



Fig. 23. A portion of the single gill of Venus, drawn as an outline plan. 

 e, d,f, mark the course of the blood, and the character of the 

 blood-channel, from the attached margin {a) of the superior 

 lamella, to the end of its course at the proximal border (b) of the 

 inferior lamella. 



Fig. 24. A second plan of the gill of Mytilus, constructed on the supposi- 

 tion that each lamella carries only a single blood-current, of 

 which the beginning is shown at d, mai-gin a, and the end at d, 

 margin b. The border, h, depicts the order of the vibratile cilia. 



[To be continued.] 



