LAMELLIBBAKCHIATA. 507 



blood is received from the veins of the gills (p) in this, as in all other 

 dimjary bivalves, by two auricles (o, o), which transmit their con- 

 tents to the single fusiform ventricle, perforated in the remarkable 

 manner just shown. In the genus Area, which is remarkable for its 

 great breadth, the ventricle itself is divided into two cavities, 

 having the rectum in the interspace. An artery is continued from 

 each 'extremity of the ventricle, which distributes the oxygenated 

 blood over the viscera, the muscular system, and the mantle. The 

 efferent vessels speedily lose the cylindrical form and expand or open 

 into large sinuses conformable with the lacunte of the viscera and 

 other organs. 



The gills are essentially internal highly vascular folds of the pallial 

 membrane, and are strengthened by series of delicate jointed fila- 

 ments which support several rows of curved vibratile cilia. The 

 respiratory currents are occasioned by the ceaseless action of these 

 cilia, and are not dependent upon any opening or closing of the 

 valves of the shell. The ciliary action is that likewise which brings 

 the nutrient molecules to the mouth, chiefly along the marginal 

 grooves of the branchial plates, where the molecules are mixed with 

 mucus and njoulded into small filamentary masses. 



In Lucina and Corbis there is only a single gill on each side : as a 

 general rule there are two. Each gill consists of two membranous 

 plates, continued into each other at the free margin of the gill {Jig. 

 193, h) ; the contiguous plates of the two gills are continuous at the 

 base or bottom of the branchial interspace, where they are fixed : the 

 basal border of the plate forming the opposite side of the gill is free 

 ■where it extends along the base of the foot, and is continued into the 

 base of the inner gill of the opposite side, behind the foot, in the 

 fresh-water muscles, as at e. The two plates of each gill are united 

 together at pretty regular intervals in the direction of their breadth 

 by transverse septa or bars {Jigs. 190, i, 193, c) so as to include 

 canals running transversely to the gill-plates. According to the 

 course of the current of water through these interlaminar canals they 

 commence by the small shts or pores along (e) the groove on the free 

 margin, in the branchial chamber, and terminate by the wider 

 openings at the fixed margin of the gill in the anal chamber. 



Fig. 190 shows a magnified section of the free margin of the two 

 gill-plates and their sustaining laminae, showing the terminal cilia 

 on the prominent border of each, the processes {a, a) by which con- 

 tiguous bars of the same plate are joined together, and the slips or 

 septa (t) uniting the bars of the opposite plates. The way in which 

 the contiguous bars are united is shown at D. In the more highly 

 magnified section, y shows the thick border, and h the thin border of 



