CONCIIIFERA. 319 



for the passage of the foot. In other groups the mantle is 

 entirely open, as in the Ostreiclce, in which case there are no 

 distinct pallial orifices, and there is free ingress and egress 

 for the water between the disunited leaves of the mantle. 

 In those families in which siphonal tubes are present, the 

 sinus of the pallial impression is more or less conspicuous, 

 and the gills of each side are united posteriorly and pro- 

 longed into the branchial siphon; while in those tribes in 

 which the siphons are wanting, the pallial line is simple, 

 and the branchial plates are distinct on each side, and are 

 not united behind or extended into the branchial tube. 



Kespiration in the Conchifera is always aquatic, and is 

 performed by means of gills, which assume the form of two 

 vascular and membranous lamellae on each side of the body, 

 covered by the lobes of the mantle, and which vary in struc- 

 ture in different tribes and families. As first shown by Dr. 

 Sharpey, and afterwards confirmed by Mr. Albany Hancock 

 and Dr. T. Williams, the water flows in by the pedal orifice 

 and branchial siphon into the pallial cavity, passes through 

 the gills, and having parted with its oxygen, is conveyed 

 backwards between their lamina?, and is carried out through 

 the upper or exhalent orifice with effete and fecal matter 

 from the intestine. The lower or branchial tube is hence 

 termed the inhalent or respiratory siphon, while the upper 

 or anal has been called the exhalent or excretory siphon. 

 Mr. William Clark denies the existence of any intercommu- 

 nication, and maintains that the two tubes are indepen- 

 dent, and that the water, admitted by the pedal orifice and 

 branchial siphon, is expelled indiscriminately from those 

 apertures. 



The foot is the organ of locomotion, and appears to be 

 of great importance in the economy of these animals ; it is 



