702 



Zf )OLOfi V 



Fir. ^84— ABpergillum, 



(After Sowerby.) 



the foot is sometimes provided with a levator muscle, particularly 

 well developed in Nucula and its allies. 



Immediately posterior to the foot a byssus-gland is fre- 

 quently found : it secretes a silky substance in the form of 

 threads Avhich 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. 585, 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 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 adjustors and also as retractors 

 of the byssus. It must be borne in mind 

 that the definite byssus j ust described is not 

 homologous with the provisional byssus of 

 Anodonta (p. 693) which lies in front of the mouth. 



The gills or ctenida are two in number, right and left. Each 

 consists of a horizontal axis bearing two rows of filaments, outer 

 and inner, which are outgrowths from it. In the Protobranchia 

 (e.g. Nucula) the filaments are short, compressed, and free from one 

 another (Figs. 586, g, 

 and 587, A). In Amu- 

 sium (B) the gill-fila- 

 ments are much elon- 

 gated and thread-like 

 instead of triangular. 



In the common Ark- 

 shell (Area, C) a great 

 change is seen. The 

 gill-filaments are deli- 

 cate and somewhat flat- 

 tened 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 fiexure 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 (see Fig. 588), 



Fig. 086.— Mytilus ednlis, attached by byssus (i>. 

 to a piece of wood. F. foot : S, exhalaiit siphon. 

 (From the Camhridge Natural Hhiory.) 



