291 



CHAPTEE IX. 



MOLLtrSCA. 



The MoUnsoa form a large class of animals, to which belong 

 Snails, Slugs, Oysters, Cookies, 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, &o.) have a pulmonary 

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

 &c.) have gills or branchise, 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 ; and there 

 are numerous terrestrial species, but the land genera are few. In- 

 ternally Mollusca are provided with a distinct heart and alimen- 

 tary 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 



