SNAILS AND MUSSELS 



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ant. odJucTor 



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FiC. 260. — Mussel shell: i, the outer surface, 

 showing lines of growth; 2, the inner surface, with 

 scars made by the attachment of muscle. 



(Fig. 259) or lies at rest on the bottom with its shells partly 

 open. When they gape wide apart, the mussel is usually 

 dead. 



Growth of the sheU. — The shell is produced by the outer 

 surface and border of the mantle. Additions are built on at 

 the edge and there the shell is newest and thinnest. In the 

 larger mussels there is a dark horny covering or periostracum 

 on the outside, then a thick limy layer, and finally the pearly 

 lining. The outer surface of the shell is marked by lines or 

 rings (Fig. 260), usually more conspicuous than the similar 

 ones on snail shells. These heavy lines or ridges seem to 

 indicate periods when cold or lack of food or some other cause 

 lowered the activity of the animal and kept it from secreting 

 very much new shell. They signify slow shell growth and the 

 spaces betv/een them mark periods of rapid shell growth. 

 Both hard and easy times in the mussel's life are recorded by 

 these lines of growth. 



Form of the body. — These headless mussels rely entirely 

 upon currents of water to bring them food and oxygen. Such 

 currents are created by the movements of microscopic cilia 

 borne upon the mantle and body surfaces. By their motion 

 water is brought into the incurrent siphon, and distributed 

 through the gills and toward tha mouth, carrying minute food 



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