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CHAPTEE IX. 



MOLLUSCA. 



The MoUusca form a large class of animals, to whicli belong 

 Snails, Slugs, Oysters, Cockles, Mussels, Octopi, and Sepias, as 

 well as many extinct forms, such as Ammonites. 



Mollusca are generally provided with a shell formed by part 

 of the animal known as the mantle : this shell is nearly always 

 external, but in some few it is buried under the skin (Slugs). 

 All mollusca are unsegmented animals, and bilaterally sym- 

 metrical in the young or embryo stage. The bilateral sym- 

 metry soon goes in the adult, and they are often much contorted, 

 especially those that lie in a spiral shell. The skin of Mollusca 

 has peculiar characteristics : one particular part — the so-called 

 " mantle " that lines the shell — has the power of secreting cal- 

 careous salts which go to make up the shell. This mantle forms 

 a cavity in the mollusc, the so-called " mantle cavity " or " re- 

 spiratory sac.'' Some molluscs (Snails, &c.) have a pulmonary 

 sac, and live upon land, breathing air ; others (Mussels, Oysters, 

 &c.) have gills or branchiae, which are bathed with the water 

 in which they live. Most Mollusca are aquatic — the majority 

 marine, a few fresh -water; and some are amphibious. The 

 terrestrial species are numerous, but the genera few. Internally 

 Mollusca are provided with a distinct heart and alimentary canal, 

 the former consisting of two auricles and one ventricle, the 

 ventricle traversing the two auricles ; both are surrounded by 

 a membrane, the pericardium. From each end of the ventricle 



