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TEXT-BOOK OF ZOOLOGY 



1. How the Animal attaches itself to the Bottom. We may have to 

 wait for hours before the mussels give a sign of life. At last the valves 

 of the shell open a little, and a white body, the hatchet- shaped foot, 

 protrudes through the gap. It extends itself more and more by degrees, 

 and slowly bores its way into the gravel. Having obtained a purchase, 

 it raises the animal upon the edge on which the valve opens (ventral 

 edge), and gradually penetrates into the gravel, until, after some hours, 

 only the pointed (posterior) end of the shell is left protruding. In this 

 condition the mussel usually remains at rest at one and the same spot, 

 and though by means of its foot it is able slowly to plough its way 

 through the ground, it is as a rule a sessile creature. 



AS. 



Jf. 



Mt. 



FKESH-WATEU MUSSEL LAID OPEN. (Natural size.) 



The soft body lies in the left (lower) half of the shell ; the right half of the mantle, which in 

 the closed mussel lies upon the right (upper) half of the shell, has been separated along the 

 dotted lines (- - -) in order to show the several parts of the body. V., Anterior, 

 H., posterior, end of the animal; Sb., hinge ; Sz. and SI., teeth and ridges of hinge ; 

 Sch., margin of shell ; Mt., left mantle lobe ; F., foot protruded ; Ms., the two right labial 

 tentacles ; atM., entrance to mouth ; K., the two right branching ; R., body ; v.S. and h.S., 

 anterior and posterior adductor muscles of shell ; A.S., places of attachment of adductor 

 muscles on right valve of shell ; E., left half of tubular afferent aperture formed by edge 

 of left mantle lobe ; A., efferent aperture. 



2. Protection. The otherwise defenceless animal is protected by a 

 strong shell which encloses the body. Total seclusion from the outer 

 world can, however, be only temporary, since the animal is obliged to 

 breathe and to take in nourishment. Seclusion of the animal at the time 

 of danger, and communion with the outside at other times, is rendered 



