MOLLUSCS 



139 



mantle. This mantle is a double fold of skin which lines and makes 

 the shell, whose folds also form a chamber in which lie the gills. 

 When undisturbed the mussel causes a constant current of water 

 to flow in and out of its mantle chamber ; this current bringing 

 oxygen and food and carrying away waste. The incoming- 

 current enters by a wide (inhalant) aperture, while the outgoing 

 one leaves by a comparatively narrow (exhalant) opening, to be 

 seen at the side of the shell opposite the foot. By the time these 

 observations have been made the mussel will probably have 

 satisfied its exploratory instinct, and will have settled down on 

 one spot, to which it fixes itself by the so-called byssus, a mass 

 of silky threads secreted by a gland which is the equivalent of 

 the mucus gland of the garden-snail, that is, of the gland which 

 secretes the trail of slime that animal leaves behind it as it creeps. 

 When we remember that the mussel cannot swim, and that it 

 lives in tidal water, the use of this power of forming anchoring 

 threads is obvious. Again, as despite the threads the mussels 

 are always liable to be carried away by a specially strong current, 

 it is obviously an advantage that they have the power of rapidly 

 weaving new threads. 



Prettier than the mussels are the scallops or clams, whose 

 gaily-coloured shells are prized by children under the name of 

 dolls' fans. In adult life the 

 scallops live in comparatively 

 deep water, whence they are 

 dredged for market, but small 

 specimens of the common 

 form (Pecten opercularis) are 

 frequent on the shore rocks. 

 The shells are of a reddish- 

 orange colour, and are almost 

 circular, each valve having 

 two projections or ears near 

 the hinge. When the valves of 

 the shell open one sees that 

 the edge of the mantle is beset with a great number of simple eyes, 

 as well as with delicate filaments. The eyes are apparently 

 associated with the power of swimming, for if alarmed the scallop 



FlG. 65. The common scallop {Pecten opercul- 

 aris). Note the eyes and the processes on 

 the mantle. 



