XII 



PHYLUM MOLLUSCA 



685 



Immediately posterior to the foot a byssus-gland is frequently 

 found : it secretes a silky substance in the form of threads which 

 serve to anchor the animal permanently or temporarily. It is by 

 means of the byssus that the Sea^-mussel (Mytilus) is attached to 

 the rocks (Fig. 595, By) : in Pinna the threads are fine enough to 

 be woven in a fabric. In Lima the threads of the byssus are spun 

 into a kind of nest in which the animals lie protected, and in species 



FIG. 597. Half transverse sections of various Pelecypoda to show the chief kinds of gill. 

 A, Nucula ; E, Amusium ; C, Area ; D, Mytilus ; E, Anodonta ; F, Poromya. 



a. aperture in branchial septum ; b. v. bk^d-vessel ; ft. foot ; t. /. inner row of filaments ; 

 i. g. inner lamina ; i. I. inner lamella ; i. I. j. interlamellar junctions ; m. mantle ; o. f. outer 

 row of filaments ; o. g. outer lamina ; o. I. outer lamella ; sep. branchial septum. (Modified 

 from Korschelt and Heider, and Lang.) 



of Modiola similar modifications of the byssus occur. In such 

 forms as Mytilus the muscles which ordinarily serve to retract the 

 foot are inserted mainly into the byssus : the latter being fixed, 

 they serve to rotate the animal in various directions, or, in other 

 words, act as adjusters and also as retractors of the byssus. It 

 must be borne in mind that the definite byssus just described is not 

 homologous with the provisional byssus of Anodonta (p. 675) 

 which lies in front of the mouth. 



