MOLLUSCA 143 



Characteristics 



1. Mollusca are mostly unsegmented and without jointed ap- 



pendages. 



2. Symmetry is fundamentally bilateral, but in the Gastropoda 



there is superposed dextral or sinistral asymmetry. 



3. The foot is usually for locomotion. 



4. The mantle is a dorsal fold of the body wall which covers the 



animal. 



5. Frequently a shell is secreted by the mantle, but sometimes the 



mantle and shell are absent. 



Natural History 



Class I. Lamellibranchiata. Clams and Mussels. — As a type 

 for study either the fresh water mussel {Anodontd) or the clam 

 {Venus) proves excellent. Both are bilaterally symmetrical and 

 have a well-developed foot. 



The Structure of the Shell. — " The mussel shell consists of three 

 layers. The outside horny layer is called the perlostracum; the 

 middle prismatic layer is formed from tiny prisms of calcium car- 

 bonate separated by thin layers of the horny conchiolin found in 

 the perlostracum; the Inner layer Is the nacre or ' mother of pearl * 

 which consists of alternate layers of calcium carbonate and conchio- 

 lin arranged parallel to the surface. The perlostracum and the 

 prismatic layers are secreted from the edge of the mantle, while the 

 nacre Is secreted from the whole of the epidermal surface of the 

 mantle." ^ 



Externally marking the shell, we find rather prominent depres- 

 sions, three or four in number, called lines of growth. Indicating the 

 number of seasons of growth. Less prominent depressions, the 

 lines being close together. Indicate the number of new edges of the 

 shell laid down by the mantle during the course of a single season. 

 There are also annual layers of nacre. 



Internal Anatomy. — In both forms we find well-developed ante- 

 rior and posterior adductor muscles. These must be cut in order to 

 separate the shells. When cut, the dorsal hinge ligament forces the 

 valves to gape open. Lining both valves we find the mantle. This 

 is adherent to the shell ventrally just Inside the edge, the point of 



1 Parker, T. J., and Haswell, W. A. 1928. Text-book of Zoology. The Mac- 

 Millan Co., London. 



