LAlttLLIBRANCHlATA. 509 



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 plates, 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. 



Although the microscopic cilia form the ordinary and constant 

 dynamical part of respiration, the function is occasionally influenced 

 by the muscles of the shell, as when the water is squirted out of the 

 siphons by a sudden shutting of the valves. The quiet and ordi- 

 nary respiratory current enters, in Anomia, at the anterior base of 

 the shell, and escapes posteriorly near the termination of the branchiae. 

 In Modiola vulgaris the current enters by the cirrigerous border of 

 mantle, and between that part and the foot : it escapes by the 

 posterior produced part of the mantle. In Mactra and Tellitia, when 

 the conjoined siphons are extended and the hyaline valve is exserted 

 from the anal siphon, the current flows in at the " inhalent " branchial, 

 or ventral siphon (^g. 188, A), and rushes out by the "exhalent," 

 dorsal, or anal siphon (ib. </) : there is no current at the pedial aperture. 

 The branchial siphon is often much dilated, so that its diameter greatly 

 exceeds the anal one, e. g., in P/iolc^s. The siphonal apertures, espe- 

 cially the inhalent one, are provided with a circle of tentacles, to 

 prevent the ingress of noxious particles. 



There is a remarkable plexus at the base of the gills, near the pe- 

 ricardium, which surrounds a distinct glandular organ in the higher 

 bivalves. It is double : each sac is elongated with glandular walls, 

 and communicates with the pallial cavity by a small prominent ori- 

 fice, usually close to the genital pores. The secretion of this body 

 abounds with calcareous particles, and it was called by Poll the secre- 

 ting organ of the shell : it is shown at Jig. 189, r. Modern analysis 

 has detected a large proportion of uric acid in these sacs, and has 

 thus determined them to be the renal organ. 



An orifice at the extremity of the foot of Solen, at the middle of 

 the foot of Cardium, and the tube situated above the pedunculate 

 anus of Pinna, admit the sea-water into a reticulate system of 



