MOLLUSCA. 15 



In the Gasteropods the sliell is sometimes wanting, as 

 in the Nudibranchs ; in others it is very small and rudi- 

 mentary in the skin of the back, as in the Jjylysiidce ; 

 and in some it only covers important organs, as in Cari- 

 naria and Testacellus. In the CMtonidiB it is composed of 

 several pieces, but in the generality of cases it consists of 

 a hollow cone closed at the apex, and more or less spirally 

 rolled. In Patella and Dentalium the cone is nearly 

 straight. In Vermetus and Scalaria it is spiral. In 

 Helioo and Coretus the spiral whorls touch. In some 

 the spiral is elevated, in others depressed ; in some 

 smooth on the outside, in others spiny or tubercular. 

 In the carnivorous tribes the aperture of the cone is 

 emarginate and produced into a calcareous tube, which 

 lodges the siphon or muscular prolongation of the mantle 

 that conveys the water to the gills ; but in the herbi- 

 vorous orders it is usually round and continuous. 



The Mollusca, like all other animals, are liable to 

 attacks of parasitic epizoa and entozoa. The Thetis 

 Jimhria is infested with the Vertumnus thecidicola of Otto. 

 Certain mussels have as a parasite Aspidogaster conchicola 

 (Bser). The intestines of Neritince are frequently full of 

 entozoa. In Neritina crepidularia are found a Lingri- 

 citula, and also an Jscaris. A species of epizoon will 

 attack Limnsei and Helices when in a sickly state, and 

 ultimately destroy them ; while a remarkable form, the 

 Disioma helicis, was found by Dr. Leidy in the pericar- 

 dium of Helix alteriiata. A small crustacean, the Pinno- 

 theros, lives parasitic in the pearl mussel; and the Phos- 

 puga atrata^ a Silphideous beetle, preys upon the common 

 snail. 



