XII 



PHYLUM MOLLUSCA 



653 



FIG. 546. Aspergillum 



(After Sowerby.) 



temporarily. It is by means of the byssus that the Sea-mussd 

 (Mytilus} is attached to the rocks (Fig. 547, By] : in Pinna the 



threads are tine 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 Vulsdla the 

 gelatinous byssus forms a sheath within 

 which the entire shell, which is of a delicate 

 character, can be retracted. 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 direc- 

 tions or, in other words, act as adjusters. 

 It must be borne in mind that the definitive 

 byssus just described is not homologous 

 with the provisional byssus of Anodonta 

 (p. 644) which lies in front of the mouth. 



The gills are found in their simplest form 

 in the Protobranchia (e.g. Nucula), where 

 they consist of a single plume-like organ 



or ctenidium (Fig. 548, gl) on each side of the body. Each 

 ctenidium is of small size compared with the gills of Anodonta, 

 and is formed of a longitudinal axis, fixed at its anterior end and 

 free posteriorly, to which are attached two rows an inner and 

 outer of somewhat triangular gill-filaments, all perfectly free 

 from one another (Fig. 549, A). 



In Amusium (B) the gill-filaments are much elongated and 

 thread-like instead of 

 triangular. In the com- 

 mon Ark-shell (Area, C) 

 a great change is seen. 

 The gill-filaments are 

 delicate and somewhat 

 flattened threads, each 

 bent upon itself into 

 the form of an elongated 

 U, and therefore con- 

 sisting of a proximal or 

 fixed limb and a distal 

 or free limb. The flexure 

 takes place in such a 

 way that the free limb 

 is external in the outer 



row of filaments, internal in the inner row. Adjacent filaments 

 are loosely united by groups of large interlocking cilia (Fig. 550), 

 placed at regular intervals, and in this way the outer and inner 



FIG. 547. Mytilus edulis, .attached by byssus (#.) 

 to a piece of wood; F. foot; S, exhalant siphon. 

 (From the Cambridge Natural History.) 



