MOLLUSCA PROPER. 179 



As already said, the margin of the mantle leaves on the 

 shell a distinct impression the " pallial line " and, by inspec- 

 tion of this, important conclusions can be drawn in any given 

 case as to the mode and life of the animal. In certain shells, 

 namely, the pallial line (Fig. 81, 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. 81, 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 spe- 

 cial muscles by which they could be withdrawn within the shell. 

 This difference expresses a real distinction among 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 

 composed generally of tubular rods (Fig. 80, b) 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 effete 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, an$ then reaches the 

 mouth (Fig. 80, s s), 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. 80). They are 

 often very small, and then they leave no traces of their ex- 

 istence 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 



