BIVALVE MOLLUSCS 331 



and pulls the shell open. Running from one adductor scar to 

 the other is a curved pallial line, marking the attachment of the 

 pallium or mantle to the shell. In a case like this, where the 

 line is unbroken by any indentation, it is integropalliate, a point 

 which will be referred to later when other bivalves are considered. 



Both shell and contained animal are bilaterally symmetrical, a 

 fact which is expressed as regarding the former by using the term 

 equivalve. Each valve is in fact a mirror-image of the other, but 

 in itself does not exhibit bilateral symmetry, i.e. is inequilateral. 



The Mussel is in the habit of remaining obliquely buried in 

 the mud with its hinder end projecting, and examination of an 

 aquarium specimen in this position will show two openings, one 

 above the other, between the mantle lobes (fig. 192). Water- 

 currents continually set into the lower or inhalent aperture, 

 serving the double purpose of carrying food to the mouth and 

 oxygen to the breathing-organs, while other currents as con- 

 stantly flow out of the upper or exhalent aperture, taking with 

 them the various forms of waste matter. The inhalent aperture 

 is fringed with sensitive tentacles, and if these are touched the 

 shell at once closes, an arrangement which is obviously protec- 

 tive. The Mussel, therefore, is able to feed, breathe, and get rid 

 of waste, with most of its body concealed from observation. 



After removal of the shell (fig. 192) it will be found that the 

 mantle-lobes are not united together except between the two 

 apertures just described. If one of them be turned back other 

 parts come into view, and the first thing to determine is which 

 is front and which back end. A distinct head will be looked 

 for in vain, and its absence is one of the characters of this class, 

 which sometimes receive the name of " headless " Molluscs 

 {Acephala, from Gk. a, without ; kephalon, a head). This 

 cannot be regarded as a primitive feature, and there is good 

 reason to believe that the bivalves are descended from forms 

 which possessed a distinct head, the dwindling of which has 

 been brought about by a sluggish mode of life and dependence 

 as regards food upon minute organisms brought to the mouth 

 by water currents. The mouth will be seen in the Mussel as a 

 wide slit just behind one of the adductor muscles, at the end 

 further from the inhalent and exhalent apertures, which thus mark 

 the hinder end of the animal. There is a complete absence of 

 anything in the way of jaws, but a pair of soft leaf-shaped bodies, 



