Respiration in Invertebrate Animals. 41 



brane. The membranous border is drawn out to a considerable 

 length ((/) beyond that part of the gill whieh is apparent to the 

 naked eye. By this extension of the aetive branehial surface, the 

 action of the aerating current on the blood is jirolonged. In no 

 single uistance of the peetinibranehiate gill are the cartilages of 

 the lanunie so developed as in this family. It is here that the 

 true structure and office of these singular elements of the bran- 

 chial mechanism may be most advantageously studied. In the 

 branchia of Bucci/ium they arc straight^ sword-shaped blades 

 (tig. 7) ; they are skilfully fitted into the dorsal or thick edge of 

 the lamina (fig. 4 a,f); they act the part of beams, or arms, 

 whereon is hung the sheet of the leaflet {a, g). Without them, 

 the latter could not by any means be held in situ ; that is, could 

 not be maintained at that degree of expansion essential to the 

 proper and adequate contact of the blood with the water. To 

 the bases of these cartilaginous beams, muscles (fig. 14 c, d) 

 are attached, which are capable of influencing the entire leaf. 

 Under their action it may be either stretched lightly or folded 

 together like a closed fan. The flat surface of the branchial 

 lamina in Buccinuin is always unplicated. 



In the fresh specimen it may be seen, with perfect clearness, 

 that it is traversed by waving parallel blood-channels (fig. 4: d,d). 

 In no instance among the Pectinibranchs is it more easy to con- 

 vince oneself that these vessels loop at the free flexible margin 

 (c) of the leaf than in Buccinuin (fig. 5). The laminse are con- 

 siderable in superficial area (a, g), exhibiting a surface equalling 

 a tenth of a square inch. The vessels [d) are prominent, being 

 readily traced by ^ an inch power. The walls display a granu- 

 lated character (fig. 5 g, g), in consequence of the contents of 

 the epithelium. This circumstance individualizes each blood- 

 channel most clearly. The flat surface of the leaf is also covered 

 by a flat, scaly, polygonal epithelium (fig. 6), the cells of which are 

 filled with minute granules, and armed with short cilia. Along 

 the base or fixed border of each leaf run two large vessels 

 (fig. 4 c, c), one of which is afi'erent, the other is efferent. Thus, 

 in brief, is written the anatomical history of this remarkable 

 organ. It is at once evident that the key-stone of this structure 

 is the beam of cartilage, whieh imparts strength and rigidity to 

 the dorsal margin of the slender sheet; and, further, constitutes 

 a point of attachment to a system of muscles, by which, as by a 

 lever, the entire apparatus may be extraordinarily furled and 

 unfurled, and otherwise variously controlled. 



Another example of the iMuricidan gill may be described, 

 in order to show, that in two closely allied species of the same 

 genus a striking difference of structure may occur in one and 

 the same organ. 



The apices of the laminae in the gill in Purpura lapillus arc 



