410 MARINE INVERTEBRATES 
Any large clam will serve for dissection. In the neighborhood 
of New York the edible species known as the “ Little Neck” clam 
(Venus mercenaria), or the larger Mactra of the sandy shores, will 
answer the purpose very well. In Florida, Callista, Venus, and Car- 
dium are all available. On the west coast the large, heavy Tivela 
crassatelloides is probably the best subject. The oyster (Ostrea) 
is a degenerate type; and the scallops (Pecten) and the mussels 
(Mytilus and Modiola) have certain special modifications of their 
organs which might prove confusing, so these genera are not 
recommended for dissection and study at first. 
Boiling is apt to shrivel and distort the soft, fleshy animal of 
the bivalves, and it is far better to examine a fresh specimen. 
The surest way of opening a clam without injuring the animal is 
to break one of the shells by sharp taps of a hammer, using great 
care not to lacerate the body within by a too vigorous assault. 
Pick off the broken pieces after having separated them carefully 
with a knife from the mantle margin, to which they cling, and 
after having cut through the tough adductor muscles as close to 
the shell as possible. The subject for anatomical study is then pre- 
pared “upon the half-shell.” 
s THE MANTLE 
The mantle is generally very thin, often a fleshy film of the 
finest tissue, and adheres to the inner side of each valve. The 
outer rim of the mantle is thickened and free, i-e., is not attached 
to the shell. This free portion is capable of slight extension 
beyond the margin of the shell when the valves are opened and the 
animal is off guard. In many genera the mantle edge is highly 
ornate, being waved, crenulated, or fluted, or is beset with several 
rows of papille, and is often richly colored. Mr. Hickson, a nat- 
uralist, who traveled in Celebes, says that the brilliant coloring 
of the mantle margins of mollusks contributed largely to the 
extraordinary color-effects upon the coral reefs. 
It will be seen that the mantle entirely incloses the animal at 
the back and sides, just as the cover of a book incloses the pages 
or printed portion. At the posterior end of the animal there is 
usually a point—or perhaps two points—at which the flaps of 
