258 Dr. T. Williams on the Mechanism of Aquatic 



the presence of these tubes, the pectinations are obvious, as in 

 Cardium, Solen, Pecten, Thracia, &c., when the tubes are large, 

 invisible to the naked eye when they are small. The cross 

 pieces tie together the longitudinal at regular intervals. The 

 latter run with the tubes, and divide them from one another, the 

 former cross them. If the transverse pieces were so thick and 

 large as to fill up the tube, and interrupt its continuity, the ex- 

 currents of water of course would be arrested, and the function of 

 the gill would be suspended. It is far otherwise. They traverse 

 the tubes in form of cords. Their extremities are attached to the 

 opposed points on the horizontal sides, — to those very lines 

 along the sides of the tubes at which the chemical act of breath- 

 ing is passive. From this arrangement there flows this most 

 beautiful result : the water, having permeated the lamella and 

 gained the interior of the tubes, in its course towards the ex- 

 current siphon, is made to keep continually in contact with the 

 branchial bars. By this simple arrangement, the two sides, in 

 fact the four sides, of each individual blood-carrying bar are 

 persistently embraced by a moving current of the respiratory 

 element ! If the cross cords did not exist, every drop of water 

 which entered the tube would collect at the most depending side, 

 and flow out as a useless and unused stream. In the oeconomics 

 of nature, the subtlest oeconomist may well marvel at her care ! 



The long pieces of this interlamellar framework are capable 

 of shortening the length of the water-tubes, the cross pieces of 

 diminishing their diameters. These actions impel, interrupt, 

 facilitate, &c. the breath-giving currents. While they com- 

 plicate the branchial machinery, they double the certainty of the 

 process; they provide against accidents; they preserve in the 

 required position the slender, tender, beautiful parts of which 

 the apparatus is composed. 



Endless diversities occur, in different species and genera, in 

 the size, the figure, the visibleness, &c. of this interlamellar 

 framework. In no single instance is there observable the 

 slightest departure in principle of structure, in intention, in 

 purpose, from the typal plan unfolded in the preceding de- 

 scription. 



The cilia-bearing epithelium (PI. VI. fig. 6 b) of the branchial 

 lamellae in the conchiferous bivalves is well known. It has been 

 well described by trustworthy observers, from Leeuwenhoek to 

 Quekett. The cilia in all cases are distributed in rows on the 

 bars (PI. VII. fig. 15 ; PI. VIII. figs. 20 & 22). There are two 

 rows on each external hemi-cylinder of each bar. On the external 

 aspect of each bar, therefore, there arefour lines of cilia (PL VII. 

 fig. 10 h, a). They drive two currents in intersecting directions. 

 On the interna aspect of each bar, that namely which faces the 



