VENUS MERCENARIA 13 



posterior margin of the visceral mass. Any material, then, which 

 touches this surface, instead of being taken toward the mouth, 

 tends to be forced in the opposite direction. Immediately on 

 touching the wall of the visceral mass, the fine particles are 

 cemented together by an abundant mucus, as on the gills. When 

 much carmine or mud is used, a large ball of it is collected at x, 

 It will be noticed that this region lies directly in the path of the 

 incoming stream of water from the branchial or lower siphon; and 

 at first sight it would seem that from this position there could be 

 no means by which it could escape from the mantle chamber. 

 Clams undisturbed in the bottom, however, from time to time 

 may be seen to discharge a strong jet of water from both siphons. 

 This habit of many lamellibranchs is better shown in Mya. When 

 these clams are kept in a bucket of water over night, the floor will 

 be wet for many feet around it in the morning, and indeed one 

 may at any time when they are so kept, see them violently close 

 the shell by contracting the adductor muscles, thus emptying the 

 mantle chamber by throwing a strong jet out of both siphons. 

 This peculiar habit of. all lamellibranchs which have been observed 

 is, without doubt, for the purpose of removing masses of material 

 which the animal can not use as food. 



This is not the only means of discharging undesirable material 

 from the mantle chamber. If the entire body be removed, leaving 

 only the mantle lining the shell on one side, it also will be found 

 to be ciliated. In this case, as illustrated in figure 5, everything 

 is swept downward toward the free edge of the mantle, and falls 

 into a line parallel with the edge, and is then directed backward. 

 Particles which may fall on the extreme edge are also passed into 

 this well marked stream. Everything is directed backward, but 

 can not be carried out of the incurrent siphon against the stream 

 which is entering through it. In a little bay beneath the base of 

 the siphon, where it is out of the current, the material is collected. 

 By the contraction of the adductor muscles, and the resulting 

 emptying of the mantle chamber, as described above, this col- 

 lected mass is expelled. 



But, in spite of the activities of these two surfaces, which tend 

 to rid the body of material not fit for food, it is evident that, if 



