196 INVERTEBRATE ANIMALS. 
The valves are shut again by the contraction of the adductor 
muscle or muscles. 
As already said, the margin of the mantle leaves on the shell 
a distinct impression—the “pallial line ”—and by inspection of 
this, important conclusions can be drawn in any given Case as 
to the mode of life of the animal. In certain shells, namely, 
the pallial line (fig. 101, 1) is unbroken or “entire,” and in these 
the-mantle-lobes were either quite free, or if attached to one 
another and drawn out into respiratory tubes, these were not 
furnished with special muscles by which the tubes could be 
retracted within the shell. In other bivalves, on the other 
hand (fig. 1o1, 2), the pallial line is indented to a greater or less 
extent, showing that the mantle-lobes were more or less united 
to one another, and were drawn out into long respiratory tubes 
or siphons, which were furnished with special muscles by which 
they could be withdrawn within the shell. This difference ex- 
presses a real distinction amongst the bivalves, due to their 
mode of life. In all alike, the respiratory organs are in 
the form of membranous leaf-like gills, of which there are 
generally two on each side of the body. The gills are com- 
posed generally of tubular rods (fig. 100, 4) richly supplied with 
blood-vessels, and covered with vibrating cilia. For the proper 
maintenance of respiration, however, it is necessary that the 
gills should be constantly supplied with fresh water. In those 
bivalves in which the animal is free and the mantle-lobes not 
attached to one another, this is effected without any special 
mechanism. In those forms, however, in which the animal 
lives buried in the mud and sand, and the mantle-lobes are more 
or less completely united, there are two orifices, one of which 
admits fresh water, whilst the effete water is got rid of through 
the other. These orifices, in the shells just spoken of, are ex- 
tended into two long tubes which are known as the “ respira- 
tory siphons.” The water passes in by one siphon, is swept 
over the surface of the gills, and then reaches the mouth (fig. 
100, s 5), when it is returned in the opposite direction to escape 
by the other siphon. The same current of water, therefore, 
both carries oxygen to the gills, and serves to convey food to 
the mouth. The two siphons may be quite distinct from one 
another, but they are very often united together so as to look 
like ‘a single tube (fig. 100). They are often very small, and then 
they leave no traces of their existence in the dead shell; but 
