MOLLUSOA PROPER. 199 



important conclusions can be drawn in any given case as to the 

 mode of life of the animal. In certain shells, namely — (e.g., the 

 Scallop, the Fresh-water Mussel, &c.) the pallial line 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. 138) 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 could be withdrawn within the shell.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 expresses 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 usually two on each side of the 

 body. The gills are composed generally of tubular rods (fig. 137, V) 

 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, while the efi'ete water is got rid of through the other. These 

 orifices, in the shells just spoken of, are extended into two long 

 tubes which are known as the "respiratory siphons." The water 

 ■passes in by one siphon, is swept over the surface of the gills, and 

 then reaches the mouth (fig. 137, o), 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. 137, s s). They are often very small, and 

 then they leave no traces of their existence in the dead shell ; but 

 when they are very long, they are furnished with muscles to retract 

 them within the shell, and it is the scar left by these muscles which 

 causes the pallial line to be indented. This indentation, therefore, 

 .as seen in the dead shell, is an indication that the animal possessed 

 long retractile respiratory siphons, and lived, therefore, most prob- 

 ably embedded in sand or mud. 



There is always a distinct heart (fig. 139, h) composed of two or 

 three chambers, and in all cases acting as a mere arterial heart. 

 That is to say, the heart propels the aerated blood derived from the 



