10 



and above by the viscera along the line below C. st., the 

 mesosoma mes. or lower middle portion of the body and 

 the adductor muscle; this npper gill cavity W cav., ex- 

 tends from h to c and is continuous with the cloaca CI, 

 which is the space between the upper free folds of the 

 mantle from b to c. 



The physiological significance of this arrangement of 

 parts will be understood by noting the directions in which 

 the arrows are supposed to be moving. As the living 

 oyster lies fixed at the bottom of the water with the free 

 edges A of the valves slightly separated, the minute cilia 

 or hair-like processes with which the gills and palps are 

 covered, are constantly in rapid vibratory motion ; this 

 peculiar movement over the gills and palps causes a cur- 

 rent of water to travel from without into the inferior gill 

 civity /. cav., a part of which passes on to be carried by 

 the cilia of the palps towards the mouth for the sake of 

 the microscopic food it may contain in suspension ; after 

 this has been gleaned out, the water probably passes back 

 to the inferior gill-cavity between the pendulous lobes of 

 the outer and inner pairs of palps. The fate of the water 

 which passes through the gills is entirely different, though 

 the gills seem to take part in carrying food towards the 

 palps and mouth, a phenomenon which may be readily 

 rendered apparent to the eye by scattering powdered 

 cochineal or hne meal on the gills or palps, when it will 

 be seen that the motion of the cilia on both organs causes 

 the particles to move with considerable rapidity towards 

 the mouth. The water passes up between the leaves of 

 the gills, actuated by ciliary motion, and enters the 

 upper gill cavity through a large number of openings or 

 canals between the gills can. and passes out towards the 

 cloaca C'Z, into which the vent Falso discharges its excreta, 

 and is so carried out of the animal back into the sur- 

 rounding water. The whole of the process of respiration 

 or aeration of the blood appears to be effected in the gills 

 proper. 



