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- 

 Mum are all available. On the west coast the large, heavy Tivela 

 crassatelloides is probably the best subject. The oyster (Ostred) 

 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." 



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, cre'nulated, or fluted, or is beset with several 

 rows of papillae, 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 



