32 LIFE ON THE EARTH. 



various contrivances are employed to adapt them to 

 the specific gravity of the liquid. Among those not 

 so adjusted are a large portion of the shell-covered 

 Mollusca, which by reason of the weight of the shell 

 are for the most part collected on the beds of lakes, 

 rivers or the sea. Within reach of the tidal agitation, 

 where the water is well charged with air, several races 

 of Mollusca attach themselves as the young oyster 

 by its lower shell fixes itself permanently and fur- 

 nishes support to others. Anomia is attached by 

 a sort of plug, Pinna and the young Mytilus by a 

 byssus spun through the agency of a muscular organ 

 called the foot, which in all the races mentioned loses 

 its usual function of locomotion. 



The attachment of Actinia by its broad base, and 

 of Patella by its circular mantle, is not of so perma- 

 nent a character. Perhaps this is also the case with 

 the suspensional ligament of Lingula and Terebratula. 

 More curious examples are furnished by the cusps 

 for adhesion which cover the arms of Cephalo- 

 poda 1 , both recent and fossil, and the sucking sur- 

 faces of Remora. 



Flotation is accomplished in some of the marine 



races by very obvious contrivances. In the large 



Medusidse whose figure is hemispherical, a steady 



position with the mouth downward is maintained by 



1 The Polypus of Aristotle and Homer. 



