264 MANUAL OF ZOOLOGY SECT. 



Whelks, Snails, Slugs, etc., (4) the Scaphopoda or 

 Elephant's-tusk Shells, 1 and (5) the Cephalopoda, including 

 the Cuttle-fishes, Squids, Octopi and Nautili. 



1. THE PELECYPODA 



A Fresh- water Mussel will serve as a convenient example 

 of the Pelecypoda. Fresh-water Mussels are found in rivers 

 and lakes in most parts of the world. Anodonta cygnea, the 

 Swan-mussel, is the commonest species in England ; but the 

 Pearl-mussel, Unio margaritifer, is found in mountain 

 streams, and other species of the same genus are universally 

 distributed. 



The Mussel (Fig. 146) is enclosed in a brown shell formed 

 of two separate halves or valves hinged together along one 

 edge. It lies on the bottom, partly buried in the mud or sand, 

 with the valves slightly gaping, and in the narrow cleft thus 

 formed a delicate, semi-transparent substance (;;/.) is seen, 

 the edge of the mantle or pallium. The mantle really con- 

 sists of separate halves or lobes corresponding with the 

 valves of the shell, but in the position of rest the two lobes are 

 so closely approximated as to appear simply like a membrane 

 uniting the valves. At one end, however, the mantle 

 projects between the valves in the form of two short tubes, 

 one (ex.sph.) smooth-walled, the other (in.sph.) beset with 

 delicate processes or fimbrice. By diffusing particles of 

 carmine or indigo in the water it can be seen that a current 

 is always passing in at the fimbriated tube, hence called the 

 inhalant siphon, and out at the smooth or exhalant siphon. 

 Frequently a semi-transparent, tongue-like body (//.) is 

 protruded between the valves at the opposite side from the 

 hinge, and at the end furthest from the siphons : this is the 

 1 Not further referred to in this work. 



