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NA TURA L HIS TOR Y. 



ACEPHALS. 



The acephalous molluscs can be readily recognized by the bivalve shell, 

 one valve being on the right, the other on the left side of the body, the two 

 being hinged together on the back. There are, besides, many other and 

 far more important characters common to all the group, some of which 

 will appear below, while one maybe mentioned now. In contradistinc- 

 tion to all other molluscs, the present group has no distinct head to the 

 body : indeed, the term ' Acephal' is but the Greek for 'without a head.' 

 Where the head should be there is simply a hole — the mouth — without 

 any accessories beyond four fleshy flaps, which form a sort of funnel and 

 thus increase the size of the aperture. 



To better understand the general structure of one of the Acephals, and 



the way it lives, a description of some of the features 

 of the common clam (Mya) of the New England 

 coast is given. This clam lives buried in the sand or 

 mud between tide-marks, the shell being sometimes 

 twelve, or even fifteen inches below the surface. 

 From this depth a chimney extends up through the 

 sand to the surface, and this chimney is occupied 

 by a fleshy tube, or, rather, a double tube, known 

 as the siphon, which forms the sole medium of 

 communication of the clam with the outer world. 

 Through one tube there constantly flows a current 

 of water, bringing the clam food and oxygen, — all 

 it needs to supply its wants. Just as constantly 

 through the other tube the current flows outward, 

 carrying away with it the waste of the body and 

 the eggs, which are destined to form other clams 

 like the parent. 



W hat is it that produces this constant current, 

 always taking the same course ? Cut open the clam, 

 and examine it as closely as possible with the naked 

 eye, and you will fail to see the impelling cause. 

 iMm-^im""^;:;;:,,;:;^ h takes a microscope to see it. The mechanism 

 tended to the surface of the consists of myriads of minute hairs or cilia, lining 



almost every part of the mantle cavity, each one 



*eping up a constant motion, and doing its part, no matter how small, in 



mg a current of water. When one, by the aid of a little powdered 



«, studio the currents flowing in and out of the siphon of a large 



