Respiration in Invertebrate Animals. 257 



lamella is diminished; on the other it remains the same as 

 long as the animal continues in the water. It is hydraulic-ally 

 inevitable, even without the assisting agency of cilia, that the 

 water must transude the lamella by way of its intervectal fissures 

 and perforations. This mechanism could neither be conceived 

 nor explained before the nature and office of the interlamellar 

 framework were brought under clear demonstration. It is im- 

 portant to understand, that that surface of the lamellae which 

 faces the intermediate tubes, namely the internal walls (PI. VI. 

 fig. 8 e) of the water-tubes themselves, is far less richly ciliated 

 than the external surface. The excurrent movement of the respira- 

 tory water is much aided by the action of the connective structures 

 of the bars and tubes. The intervectal pieces — those which pass 

 crosswise from bar to bar by approximating the latter — are 

 capable of stopping up the intervectal stigmata, — of suspending, 

 therefore, the act of respiration. Thus is prevented the passage 

 of irritating substances through the branchial lamella. The 

 alternate movements of the shutting and opening of the bars is 

 as important to the sieving operation of the gill, its prehensile 

 function, as the cilia themselves. The transverse intervectal 

 pieces (PI. VII. fig. 12 c, c, c; PI. VIII. fig. 19 c, c, c) consist of 

 irritable and contractile tissue. They are capable, in part, of 

 voluntary contraction. Thus, although the chemical act of 

 breathing is in itself uncontrollable, it may be interrupted by 

 the exercise of those connected parts which are subject to the 

 will. 



The skeleton of solid pieces (PI. VI. fig. 8 d, d, fig. 1 c, d, 

 fig. 2 i,j, &c.) by which the tubes and the lamella are supported 

 and held apart, exists probably in the branchiae of every lamelli- 

 branchiate mollu.sk. It constitutes a framework system, though 

 anatomically distinct from, having a mechanical action concurrent 

 with, the intervectal. At the free border this interlamellar sub- 

 stance is thin, slender, and difficultly detected by the eye, admit- 

 ting of the falling together of the lamellae and of the closing of 

 the tubes. At the opposite attached border, the interlamellar 

 substance is much thicker, coarser, and more conspicuous 

 (PI. VI. fig. 1 k, k, k) ; the parts being quite apparent through 

 the lamellae. Here, therefore, the lamellae are further separated, 

 and the tubes of the greatest diameter. This framework consists 

 of two distinct pieces, — those, first, which run parallel with the 

 vascular bars (PI. VI. fig. 1 c, fig. 2 i) ; and, secondly, those 

 which transversely connect these longitudinal pieces (fig. 1 d, 

 fig. 2j,j). The former limit the breadth of the interlamellar 

 water-tubes. The tubes are capacious when the lamellae 

 are far apart, small when they are near each other. As the 

 exterior appearance of " pectinations " in the gill is due to 



Ann. % Mag. N. Hist. Ser. 2. Vol. xiv. 17 



