34^ ZOOLOGY. 



specialized group, in the number of species and indi- 

 viduals. 



CLASS I. — La:hellibranchiata {AcepUala, Bivalves). 



General Characters of Lamellibranehs This group is 



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

 etc. By a study of the common clam {Mya 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 (in most 

 bivalves there are three teeth) and ligament (Fig. 161 C l). 

 The shells are 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, that containing the mouth of the mollusk. 

 The hinge is situated directly over the heart, and Is there- 

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

 two round muscular impressions made by the two adductor 

 muscles and the pallial impression, j)arallel to the edge of 

 tlie shell, 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 

 trough filled with water, and removing the ujDper valve, the 

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

 thickened, while within, the mantle where it covers the el- 

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

 head, or siplion, is divided by a |)artition into two tubes, the 

 upper, or that on the hinge or dorsal side, being excurrent, 

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

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

 is surrounded with a circle of short tentacles. This siphon 

 protrudes through a slit in the mantle-edge, and is very ex- 

 tensible, as seen in Fig. 161, 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 ( Unio, 

 Fig. 16,2) the two siphonal openings are above the level of 



