52 Dr. T. Williams on the Mechanism of Aquatic 



direction of the cloaca. When there exist two separate bran- 

 chial lamellse on either side, there exist two grooves ; when one, 

 only one. 



The anal chamber, then, should be defined as beginning in 

 these intra-branchial grooves, but remotely or primarily in the 

 i/zfer-lamellar water-passages of Mr. Hancock. The mode in 

 which this chamber communicates with the water-tubes between 

 the branchial lamella? is thus described by Mr. Hancock : — 

 "... the anal chamber (in Pholas crispata) was laid open, and its 

 ventral wall was seen to exhibit four longitudinal rows of large 

 orifices. These four rows of orifices, already well known to 

 anatomists, correspond to the attached margins of the four gill- 

 plates, which hang from the roof or dorsal membrane of the 

 branchial chamber ; this membrane being the ventral wall of the 

 anal chamber, the membrane, in fact, which divides the chambers. 

 These orifices lead into wide tubes, which pass between the two 

 lamina? forming each gill-plate. These inter-branchial tubes lie 

 contiguous and parallel to each other, and extend the full width 

 of the gill, being bifid within its free margin. Thus it is 

 evident that the tubes within the gill-plates communicate freely 

 with the anal chamber*." This description is exact, but it 

 should be thus qualified : In those siphonal families in which 

 the gills are united posteriorly (this is the case in Unio, Ano- 

 donta, Mactra, Cardium, Isocardia, Lutraria) and prolonged into 

 the inhalent siphon, the anal chamber is considerably more 

 capacious than in those in which the branchial plates of opposite 

 sides are distinct and ununited posteriorly (this condition is 

 observed in Pccten, Avicula, Area, Pectunculus, and Pinna), or 

 in those in which the siphons are suppressed. In all cases, the 

 anal chamber commences anteriorly (fig. 10, c) in grooves more 

 or less extended, formed, as already defined, between the plates 

 of the branchial lamellae and the side of the visceral mass and foot. 



These grooves terminate and pour their contents in a conti- 

 nuous stream into the anal chamber — a cloacal cavity common 

 to the branchiae and intestine. In all cases then the interlamellar 

 water-passages open throughout the anterior half or third of the 

 gills into the water-grooves at their bases, posteriorly directly 

 into the cloaca or anal chamber. This definition, so positive, 

 precludes all misconception. It leaves the cavities functionally 

 distinct in all genera, though in some structurally continuous. It 

 solves the problem of the Molluscan organism. The ingress and 

 egress of the alimentary and respiratory elements are so ordered, 

 that the ceconomy of the conchiferous animal, hidden in a coat 

 of mail, is rendered unequivocally clear to the understanding. 



* Ann. & Mag. Nat. Hist., Nov. 1851. 



