248 Dr. T. Williams on the Mechanism of Aquatic 



tion of the branchial bars in some species of Acephala. If a " bar" 

 be bent once upon itself (fig. 3), and if then one limb only be 

 rested upon a flat surface, the other limb will be on the same, 

 vertical plane, but on a different horizontal plane. If a second, 

 then a third bar, and so on, be placed in coincident directions, 

 the limbs will form two horizontal series or laminae, between 

 which a free undivided horizontal space will exist (e, /) ; but there 

 will also exist vertical spaces between each two adjoining bars 

 having the same vertical planes. In words this arrangement 

 is complex, in illustration simple. Now it may at first be sup- 

 posed that of mechanical necessity this must be the order in 

 which the bars are arranged in all the examples of double gills* 

 (PI. VI. figs. 1 & 7), as it is really that in which the afferent and 

 efferent limbs of the same looped bar are disposed in all instances, 

 without exception, of single (PI. VI. fig. 2) gills. But it is truly 

 the case only in a very few genera. It is so in the Mytilidse 

 (PL VIII. fig. 24). It follows that under the latter circumstances 

 the interlamellar water-tubes must be bounded by two concentric 

 walls (PI. VI. fig. 5), each wall being composed of a single hori- 

 zontal series of bars. Of this disposition another apparent 

 example is afforded in the Ostreadse ; if the disposition of the 

 loops at the free margin only be considered. In nearly all other 

 genera, known to the author, the limbs of the same looped bar 

 are placed on the same horizontal plane (PI. VI. fig. 7/). The 

 plane of the loop notwithstanding at the distal border of the 

 lamella is not horizontal, but vertical. It results that each 

 lamella is composed of a single series of bars, though the con- 

 tiguous limbs alternate in function, one conveying a centripetal, 

 the other a centrifugal current (PI. VII. figs. 9 & 11). But it 

 must be remembered that a single lamella [a or b, PI. VII. 

 fig. 11) of a double gill is not the exact equivalent of an entire 

 single gill (fig. 14). In all single gills the limbs of the same 

 bar rest on vertical planes ; those of a single lamella of a double 

 gill are placed on the same horizontal plane (fig. 12). In the 

 single gill the physical conditions are more favourable to the 

 complete aeration of the blood. The water-currents are dif- 

 ferent, not the same. It will greatly facilitate the comprehension 

 of the preceding history if now the minute anatomy of the free or 

 distal borders of the branchial lamella be carefully and accurately 

 studied. 



The structure of the extreme free edge of the lamella furnishes 

 a ready key which unlocks at once the whole mystery of the 

 branchial apparatus ; and yet this wondrous part of the organ 



* The meaning; attached in these papers to the double as opposed to the 

 single gill is afterwards explained. 



