256 Dr. T. Williams on the Mechanism of Aquatic 



most perfect mechanism. They arc not blood-channels. They are 

 elastic, fibrous structures, enacting a purely ligamentous part. 

 They derive their supply of blood from that of the branchial 

 bars. Their office is mechanical, not chemical. 



The intra- tubular structures (PI. VI. fig. 1 k, k, k, fig. 8 d, d; 

 PI. VIII. fig. 21 /, &c.) are neither less remarkable nor less im- 

 portant. Upon this interlamellar framework depend the whole 

 characters of the gill. They hold the lamella? apart at definite 

 distances. They unite closely together the loops of these lamellae 

 at the free margin (PI. VI. fig. 7 a, b); thus they close up crccally 

 the tubes at this border of the gill *. This single point of structure 

 is the pivot whereon turns the action of the gill. If the tubes 

 at this extremity were open (as suppositional^ at PI. VII. 

 figs. 9 & 11), it is hydraulically certain that the water would 

 take this course to pass from the extra- to the intra-branchial 

 cavity ; none would pass between the bars which contain the 

 blood ; the function of respiration could not proceed ; and this 

 calamity, further, would ensue — no food could be carried to the 

 mouth. Men do not value health until it is lost ! Spectators 

 see not, feel not, the perfection, the unimprovableness of organic 

 mechanism until an element is ideally removed— until some devi- 

 ation from nature's method of working is supposed ! The argu- 

 mentum ad absurdum startles by the bungle and foolery which it 

 is sure to introduce; then philosophers realize the inimitableness 

 of her certainty and refinement. 



As the proximal borders of the lamella? (PI. VI. fig. 1 a, b) are 

 separated by the whole diameter of the water-tubes, and as the 

 distal margins are fused together, it follows that these tubes, like 

 rivers, are small and shallow at their commencement, deep and 

 broad at their termination. This arrangement favours their 

 suctorial action. The water, as first explained by Mr. Hancock, 

 is undoubtedly drawn into (properly pushed into) these tubes 

 through the lamellar stigmata (PI. VIII. fig. 17/,/,/) from the 

 pallial cavity. The water is discharged from the tubes by ciliary 

 agency, which is constant (arrows in PI. VII. figs. 9, 11, 13 & 

 14). They are thus constantly being emptied. If they were not 

 refilled from without, they would become vacuous. During the 

 action of the gill, there is momentarily generated a tendency to 

 a vacuum. The pressure that is on the tubular side of the 



* In the accompanying illustrations, in several instances (PI. VII. fig. 9 

 & 11, PL VIII. fig. 21), these tubes are represented as if they were open at 

 this margin of the gill. This method of illustration was adopted only for 

 the sake of clearness, and in order that the disposition of the loops and 

 bars of each lamella may be readily understood. In nil cases, without a 

 single exception, the tubes are closed at this border of the gill by the appo- 

 sition of the loops of the two constituent lamellae. 



