SECTION XII 



PHYLUM MOLLUSCA 



THE Mollusca, like the Arthropoda, form one of the chief divisions 

 of the animal kingdom, both as regards diversity of organisation 

 and number of genera and species. They are sharply distinguished 

 from Arthropods by the absence of segmentation, and by having, 

 as a rule, an exoskeleton in the form of a shell, usually external, 

 sometimes internal. An enumeration of the Classes of the Phylum 

 will serve to give some notion of its extent. 



Class 1. PELECYPODA, including the bivalved shell-fish, such 

 as Mussels, Cockles, Oysters, &c. 



Class 2. AMPHINEURA, including the Chitons and their allies. 



Class 3. GASTROPODA, including the univalved shell-fish, such 

 as Periwinkles, Whelks, Snails, Slugs, &c. 



Class 4. SCAPHOPODA, including the Tooth-shells. 



Class 5. CEPHALOPODA, including the Cuttle-fishes, Squids, 

 Octopods, and Nautili. 



CLASS I. PELECYPODA. 



1. EXAMPLE OF THE CLASS THE FRESH-WATER MUSSELS 



(Anodonta and Unio). 



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. 



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The Mussel (Fig. 572) 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 (m.) is seen the edge of the mantle 

 or pallium. The mantle really consists 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 



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