MOLLUSCS. 81 



clam, noting their size and velocity, and then turns to his microscope, and 

 sees the minuteness of the means involved, he is sure to receive a realizing 

 sense of the greatness of small things. 



The water in the mantle cavity flows over the two pairs of broad, leaf- 

 like gills, conveying oxygen to the blood, and thence continues its course 

 to the mouth, which selects the microscopical animals and plants so abun- 

 dant in the water of the sea-shore, and swallows them. The gills just 

 mentioned, two in number on either side of the body, arise from the angle 

 of the mantle cavity, where the mantle springs from the body. In the 

 oyster the gills are known as the ' beard.' 



Others of the acephals differ considerably from the clam in many 

 points. Some lack the siphon, while others have it but slightly developed ; 

 some live a life of freedom, wandering at will over the bottom of sea or 

 pond ; and many have the power to open or close the valves of the shell 

 to a far greater extent than the clam. From one valve to the other pass 

 one or two strong muscles, which, when they contract, draw the two halves 

 of the shell. When they relax, the valves fly apart ; but this is effected, 

 not by muscular action, but by a spring at the hinge. The foot in the 

 clam is small, and capable of being only slightly protruded ; but in many 

 forms it is much larger and more muscular, and is in fact as well as in 

 name an organ of locomotion. 



As they are usually arranged, the oysters are the lowest of the acephals, 

 but scientific and popular estimation here differ considerably. Oysters are 

 too well known to need illustration or extended description. The shells 

 possess not the first element of beauty, but are extremely irregular in 

 shape, — a fact which makes it difficult to say how many species there 

 are. An oyster will start and grow for a while, and produce one style of 

 shell, and then suddenly the whole tenor of life will change, and the shell 

 will take on a new set of characters. Still it is now pretty well settled 

 that all the oysters upon our east coast — whether short and broad or 

 long and narrow, whether smooth or fluted — all belong to one species, 

 while both Europe and our Pacific coast are each blessed with two species. 

 Others occur in various parts of the world. 



In times not very remote our oyster existed in abundance from the 

 Gulf of St. Lawrence along the coasts of Maine and Massachusetts, and 

 so to the Gulf of Mexico. At various points north of Cape Cod there are 

 shell-heaps which mark the spots where, before the discovery, the Indian 

 had his feasts, and in most of these heaps oyster-shells are found, occa- 

 sionally, as at Damariscotta, Me., in great abundance, thus showing the 

 former existence of native beds in close proximity. Even since the settle- 

 ment by the whites beds existed at various points in Maine and on the 



