686 



ZOOLOGY 



SECT. 



The gills or ctenidia 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. 596, g, and 597, A). In 

 Atnusium (B) the gill-filaments are much 

 elongated and thread-like instead of trian- 

 gular. 



In the common Ark-shell (Area, G) 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 consisting of a 

 proximal or fixed limb and a distal or free 

 limb. The flexure takes place in such a 

 FIG. 598. Four giii-fiiaments way that the free limb is external in the 



of BXytilus. c.j. ciliary , r*-\ , i j_r 



junction ;/. filaments. (From outer row of filaments, internal in the inner 

 Natural His- TQW Adjacent filaments are loosely united 

 by groups of large interlocking cilia (see 

 Fig. 598), placed at regular intervals, and in this way all the outer 

 and all the inner limbs of the filaments are respectively joined to- 

 gether so as to convert each longitudinal row of U-shaped filaments 

 into a double plate, fairly coherent unless the ciliary junctions 

 are forcibly vm 



separated. In ix vn VI 



this way the 

 single c t e n i - 

 dium of Nucula 

 has given place 

 to two plate- 

 lik e simple 

 laminae, each 

 formed of an 

 outer and an 

 inner lamella : 

 the inner 

 lamella of t h e 

 outer and the 

 outer lamella of 

 the inner 

 laminae are 

 united along 



their dorsal edges, the line of junction representing the axis of the 

 ctenidium : the outer lamella of the outer and the inner lamella 

 of the inner laminae are free dorsally. 



In Mytilus (Fig. 597, D) the gill is strengthened by the develop- 

 ment of delicate non- vascular bars or interlamellar junctions 



FIG. 599. Dissection of Poromya. I, anterior palp; II, foot; 



III, lamella on branchial septum ; IV, valve of branchial aperture ; 



IV, anal siphon ; V, posterior adductor ; VI, posterior retractor of 

 foot ; VII, heart ; VIII, ovary ; IX, branchial septum ; X, anterior 

 adductor. (From Pelseneer.) 



