2aa zuoLoo Y. 



specialized group, in the number of species and indi- 

 viduals. 



CLASS I. — LAiiELLiBEAKCHiATA {AcepJiala, Bivalves). 



General Characters of Lamellibranehs — This group is 



represented by the oyster, clam, mussel, quohog, scallop, 

 etc. By a study of the common clam ( J/z/a arenaria Linn. ) 

 one can obtain a fair idea of the anatomy of the entire class, 

 as it is a homogeneous and well-circumscribed group. The 

 clam is entirely protected by a pair of solid limestone shells, 

 connected by a hinge, consisting of a large tooth Cin most 

 bivalves there are tliree teeth) and ligament (Fig. 155 C l). 

 The shells arc equivalve, or with both valves alike, but not 

 equilateral, one end (the anterior) being distinguishable from 

 the other or posterior, the clam burrowing into the mud by 

 the anterior end, thtit containing the mouth of the mollusk. 

 The hinge is situated direct])- over the heart, and is there- 

 fore dorsal or hoemai. On the interior of the shells are the 

 two round muscular mpressions made by the two adductor 

 muscles and the pJliul imprcssio.;, parallel to the edge of 

 the sh'jll, made by the thickened edge of the mantle. On 

 carefully opening the shell, by dividing the two adductor 

 muscles, and laying the animal on one side in a dissecting 

 trougli filled witli water, and removing the upper valve, the 

 mantle or body-walls will be disclosed ; the edge is much 

 tliickeued, "\\liile within, the mantle where it covers the el- 

 liptical rounded body is very thin. The so-called black 

 liead, or siplion, is divided by a partition into two tubes, the 

 uj)per, or tluit on the hinge or dorsal side, being exCurrent, 

 the lower and larger being incurrent — a current of sea-water 

 laden with minute forms of life passing into it. Each orifice 

 is sun-ounded with a circle of short tentacles. This siphon 

 is a tubuhii })rolongation of the mantle-edge, and is very ex- 

 tensible, as seen in Fig. 155, A ; it is extended, when the 

 clam is undisturbed, from near the bottom of its hole to the 

 level of the sea-bottom. In the fresh-water mussel ( Vn io. 

 Fig. 156) the two siphonal opcniugs are above the level of 



