OMNIVOROUS MOLLUSCS, INSECTS, AND CRUSTACEANS 249 



tinually stream, and a smaller one above, from which water 

 constantly flows out. The ingoing currents are rich in dissolved 

 oxygen, which is used for breathing, and also bear along innumer- 

 able minute plants and animals, which are conducted forwards to 

 the mouth and serve as food, passing on either side along the 

 groove between the two labial palps. The outgoing currents, 

 on the other hand, sweep away all the various products of waste. 

 The flow in both cases is produced and its direction determined 

 by the action of the cilia which line the mantle-lobes and cover 

 both gills and labial palps. The gills, indeed, are as much 

 current-producing organs as anything else, and appear to have 

 been greatly specialized in the Mussel and many other bivalves 

 for the purpose of doing this particular kind of work. As ciliary 

 action is independent of the will, there is not so much scope for 

 intelligence as in, say, a garden snail, and the Mussel is a 

 sluggish creature with dwindled head, living in a happy-go- 

 lucky way by keeping its mouth open and taking in what the 

 Fates provide. 



Many bivalve Molluscs are able to feed and breathe when 

 much more deeply buried in mud or sand than is the case with 

 the freshwater Mussel. For it often happens that the two open- 



Fig. 462. — Scrobicularia, with siphons and foot protruded 



ings serving for the entry and exit of currents are placed at the 

 end of tubes of greater or less length, and the animal gets on 

 quite well if the tips of these just protrude at the surface. A 

 good example of this is found in a little oval form {Cyclas corned) 

 which is abundant in fresh water. If a number of these are placed 

 in a bottle of water and left alone for a short time they will 

 present a very interesting sight, for from the hinder-end of each 

 of them two translucent tubes will be seen projecting upwards. 

 These are the siphons, which bear the all-important openings, and 

 close observation enables the two water- currents to be plainly 



