SECTION XII 

 PHYLUM MOLLUSCA 



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

 divisions of the animal kingdom, both for diversity of organiza- 

 tion and for 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, 

 Octopi, 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. 



The Mussel (Fig. 524) 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 



