252 Dr. T. Williams on the Mechanism of Aquatic 



craft : attempting to mimic nature^ it is lost in caricature. 

 Nature does not reach her ends by the "fusion^' of organs. 

 An existing organ is modified to fulfil a collateral purpose. A 

 solitary gill has its own peculiar characters. The component 

 vessels remaining unchanged in number and arrangement, a 

 solitary organ is rendered equivalent to a double one, by aug- 

 menting the dimensions of the passages and tubes in such a 

 manner, that the aerating element brought into relation with 

 the blood can be increased almost to any amount. Function is 

 thus intensified, while structure remains analtered. 



The loops of the vascular bars, as they project at the free 

 margin of the lamellae, are differently joined and variously 

 figured and sculptured in different genera, and frequently in 

 different species of the same genus. In Pholas (PI. VI. figs. 1 

 & 4), the free border of the double or inner gill presents two 

 rows of loops (<?,/). The plane on which the loops of the upper 

 lamella rest is horizontal, coinciding with the lengthy as opposed 

 to the breadth, of the gill. Those belonging to the lower lamella 

 of the same gill, form a row on a plane an eighth of an inch below 

 the former. Between these two projecting scalloped edges, a 

 groove (fig. 3 b) runs from one end of the gill to the other. The 

 cilia which fringe this groove (PI. VIII. fig. 24 h ; PI. VII. fig. 10, 

 a, b) are very much larger in all species than those which are 

 distributed over the bars at the plane faces of the gills. They 

 excite a vigorous current, bearing towards the mouth. Those 

 of the flat surface (PL VIII. fig. 20 ; PI. VI. fig. 6 Z», b) raise 

 streams, tending towards the free border of the gill. Both are 

 subservient to alimentation and respiration. In Pholas, then, the 

 double gill (PI. VI. fig. 1) is composed only of two lamellae, like 

 the single gill (PL VI. fig, 2) ; but in the former, each lamella, 

 is composed of two orders of bars, in the latter of one order only.; 

 The two limbs (fig. 4 b, b, c, c) of each looped bar in the former" 

 are placed on the same side of the intermediate water- tube : the 

 afferent and efferent limbs of the same bars (fig. 2 h, /), in the 

 instance of the single gill, are so opened or separated at the free 

 margin as to form respectively the opposite walls of the included 

 water-tube. The vascular loops at the margin of the double gill 

 in nearly all genera are disposed flatwise (PL VI. fig. I e,f; 

 PL VIII. fig. 2lc,b; PL VII. fig. 9 a, b, fig. II a, b), so that all 

 the loops of the same lamella form one horizontal plane. Those 

 of the single gill (PL VII. fig. 14 «; PL VI. fig. 2) are placed 

 vertically, so that the plane of each loop is separated from, 

 though parallel with, that of the adjoining loops. In the double 

 gills of the Cardiadse an exception occurs, and probably in other 

 families. The loops at the distal margin -are disposed here on 

 vertical planes (PL VII. fig. 14 a) ; but though standing verti- 



