LAMELLIBRANCHIATA. 283 



elongated amber-coloured body, called the crystalline style, is indi- 

 cated at k : it is contained in a special cyst, with its free extremity 

 protruding into the stomach ; it is peculiar to certain species of 

 dimyary bivalves, and its use is unknown. The intestine {l) forms 

 a few convolutions, and terminates in the rectum (?«), which per- 

 forates the ventricle of the heart {ii). The blood is received from 

 the veins of the gills (jt?) in this, as in all other dimyary bivalves, by 

 two auricles (o, o), which transmit their contents 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 branchial plates are essentially internal folds of the pallial 

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

 ments, which support as many series of curved vibratile cilia. The 

 respiratory currents are occasioned by the ceaseless action of these 

 cilia, and are not dependant 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. 



The two branchial lamellae of one side are usually connected with 

 those of the opposite side by their posterior extremities only ; but 

 sometimes the union is more extensive. In a few genera, as Afiatina 

 and Pholadomya the two lamellae of the same side are so united as to 

 appear like a single gill. In the Pholadomya it forms a thick oblong 

 mass, finely plicated transversely, attenuated at both extremities, 

 slightly bifid at the posterior one. A line traverses longitudinally the 

 middle of the external surface, which has no other trace of division. 

 The branchiae on each side adhere to the mantle by the whole of their 

 dorsal margin, and are united together where they extend beyond the 

 visceral mass, being separated, by the interposition of that mass, 

 along their anterior two-thirds. A narrow groove extends along the 

 free anterior margins of each gill. When the inner side of this ap- 

 parently simple gill is examined it is seen to be divided into three 

 longitudinal channels, by two ridges, containing the vascular trunks 

 and nerves of the gills. A style passed from the excretory siphon, 

 behind the conjoined extremities of the branchiae, enters the dorsal 

 channel, from which the excretory respiratory currents are discharged : 

 the middle channel is characterised by an orifice which conducts into 

 the cavity of the gill, where the ova are hatched : the third channel 

 forms the inner or mesial surface of the gill, which is not otherwise 

 divided. 



