MUSCULAR SYS 143 



generally at least two. This mi^H tlie 



pierced valve of anomia to be I nal 



body. Between the two musci he di- 



myaria, there is commonly a rough t halves 



running near the ventral margin, and se^u 3 corres- 



ponding situation in the monomyaria, which makates the 

 place of attachment of the fibres of the palleal muscle, 

 by which the projecting free margins of the mantle are 

 retracted into the shell. This marginal muscle consists 

 of numerous small fasciculi attached along this groove in 

 both valves, and spreading chiefly on the loose ventral 

 margins of the mantle. By forcibly retracting this part 

 of the mantle, these palleal muscles contract the respiratory 

 sac, so as to assist in a forced expiration of the contained 

 water, and they protect the most sensitive marginal part 

 of the mantle from being compressed and injured during 

 the closing of the valves. The currents of water which 

 are conveyed into the cavity of the mantle through the 

 respiratory orifice, and outwards through the vent, for the 

 purposes both of respiration and nourishment, are entirely 

 produced by the rapid action of vibratile cilia, which are 

 disposed in the closest arrangement around all the minutest 

 meshes of the branchiae, and cover all the fringed edges 

 of the respiratory orifice and nearly the whole inner surface 

 of the respiratory cavity of the mantle. As all other vibra- 

 tile cilia, these continue in lively activity on portions of 

 the gill or mantle which have been long cut from the 

 body of the animal. The respiratory cavity of the mantle, 

 with highly contractile muscular parietes, is often prolonged 

 to a great distance beyond the margin of the valves, es- 

 pecially in burrowing species, to reach with the respiratory 

 and anal orifices, the surface of the bed or rock in which 

 the animal is concealed. Besides the usual large adductor 

 muscles, there are frequently smaller supplementary trans- 

 verse muscular bands passing from the dorsal part of one 

 valve to the other. 



The muscular foot of the gasteropods sometimes covers 

 the whole ventral surface of the body, and sometimes ex- 

 tends only from the under surface of the neck ; it is the 

 largest muscle of the body, and that by which progressive 

 motion is effected both in creeping and swimming. In the 

 inhabitants of turbinated shells or the trachelipodous gas- 



