Respiration in Invertebrate Animals. 51 



the visceral mass, although open, form really the commencement 

 of the anal chamber. 



Mytilus and Pecten exemplify the type of this condition 

 (PL I. fig. 7 2 ). The outer plate (c, c') of each lamella is free or 

 unattached at its proximal margin. This latter is thick and 

 strong. It is composed of the large afferent and efferent trunks. 

 In Mytilus and Pecten it is not fringed by a slender membrane 

 as in Cardium and Pholas (fig. 6, e, f) and Mya, &c. The 

 gutter or channel (fig. 7 2 , f, f) formed by the attached or lower 

 lamella of the superior gill and the free or unattached or upper 

 plate, opens consequently into the extra-branchial or pallial 

 cavity in a direct manner. The water flowing along this groove 

 (fig. 10, c) does not however return into this latter cavity, 

 except under extraordinary circumstances. It is conducted in a 

 rapid course, impelled by the branchial cilia, in the direction of 

 the cloaca (fig. 10, e). The groove formed at the proximal 

 margins of the inferior, or as it is falsely called, the supplemental 

 gill (fig. 7 2 , c'), opens in like manner into the pallial cavity, but 

 on the under surface. It receives the expiratory currents of the 

 lower gill, and conveys them in form of a strong single current 

 towards the exhalent siphon. In Cardium (PI. II. fig. 9), Pholas 

 (fig. 7), Mya, Solen, Cochludesma, and Pholadomya, the two 

 plates of each branchial lamella on both sides are attached, the 

 upper to the side of the body and foot, the lower to the mantle. 



The groove bounded by the plates of the branchial lamellae is 

 divided off, therefore, in these genera by a continuous mem- 

 brane (fig. 9 /) from the pallial chamber. Mr. Hancock says, 

 that in the siphonal families this membrane forms a complete 

 partition between the pallial and anal cavities, since it extends 

 continuously from the anterior to the posterior extremity of the 

 upper plate. The author's observations have convinced him, 

 that, while in Cardium, Pholas, Solen, and Mya this membrane 

 stretches posteriorly over the cloaca in a hood- or tongue- like 

 form, it leaves between its edge and the side of the foot a 

 fissural opening through which the two cavities freely communi- 

 cate. The difference, then, between the siphonal and non- 

 siphonal families as respects the parts concerned in respiration, 

 may be defined as consisting in the degree in which the intra- 

 branchial grooves are anatomically isolated from the open space 

 of the pallial cavity. 



Such points are non-essential distinctions : while they denote 

 the existence of trifling structural varieties, they involve no 

 diversities in the methods of action. Whether partially open or 

 completely closed, the grooves (fig. 6,/) running horizontally at 

 the proximal borders of the gills and between their component 

 plates, convev the exhalent current received from the sills in the 



4* 



