BIVALVES AND CUTTLES. 267 



in correspondence with the shell whose valves it lines. At 

 the straighter side of the shell there is protruded a slender 

 white foot (/ in Fig. 77), by means of which the animal 

 slowly moves. If allowed to remain undisturbed it ulti- 

 mately anchors itself by a rope of threads, or byssus (b in 

 Fig. 77), secreted from the foot, and serving to fasten the 

 animal to the surrounding stones or shells. The byssus is 

 rapidly formed, 

 and can soon be 

 renewed if torn 

 away. At the 

 side of the shell 

 opposite to the 

 foot it will be 

 seen that the 

 two parts of the 

 mantle are fused 

 together at two 



places a little ^ ia - ^ Edible mussel (Mytilus edulis). For letters 



distant from one 



another, so that a very short tube (e in Fig. 77) is formed. 

 By placing the mussel in slightly turbid water, it is easy 

 to see that in life there is a continuous current of water 

 entering by a wide space between the halves of the mantle 

 marked i in Fig. 77, and leaving by this short tube, which 

 constitutes the exhalent aperture. The lower current brings 

 with it food particles and the oxygen necessary for respira- 

 tion, the upper current carries out the waste carbonic acid 

 and the indigestible residue of the food. 



Turn next to the little Tapes, and you will find very 

 similar conditions, save that the foot is of a different shape, 

 and the two apertures are drawn out into long siphons or 

 tubes, which can be protruded or retracted, and whose tips 

 are beautifully fringed. A little observation will show that 

 by the upper of these the water escapes, while it enters by 

 the lower. Again, while the mussel must live freely ex- 

 posed to the water, the carpet-shell, on account of its 

 siphons, is enabled to live buried in sand or mud with 

 merely the siphons protruding. Eefore proceeding to re- 

 move the upper valve of your specimens to examine the 

 anatomy, study some empty shells of the same or different 



