44 



LAMELLIBBANCHIA. 



an outgrowth of the ectoderm arises (Fig. 19 mr\ forming a fold 

 which extends anteriorly and posteriorly over the body (Fig. 21 A, 

 mr). 



As in Teredo (p. 34), the mantle extends with the shell from the 

 dorsal to the ventral side. The outer lamella of the mantle is closely 

 applied to the shell, while its inner lamella bounds the mantle-cavity 

 which has arisen with the growth of the mantle. As the shell grows 

 the mantle also increases more and more in size, and the two now 

 enclose a large part of the body (Figs. 21 B, and 31, p. 75). 



The siphons, where found, appear to develop at a very late stage. 

 They are formed from the edges of the mantle, which become closely 

 applied and fuse, the parts destined to form siphons then growing- 

 out into long, tubular structures. 



m+l-' 



FIG. 21. A and />', embryos of C 

 f, foot ; 



s cornea (after ZIEGLER). , aims ; by, byssus ; 



/. loot : ff, genital rudiment; k, gill; m, mouth; m + l, stomach and liver; mr, 

 edge of the mantle ; n, kidney ; p, pericardium ; s,, unpaired shell-plate ; s ll} rudi- 

 ment of the right calcareous shell-valve ; sd, shell-gland ; vd, stomodaeum ; iW, 

 velar area. 



Within the mantle-cavity another ectodermal fold appears on either 

 side of the body, developing from behind forward like the mantle 

 itself. This paired fold is the first rudiment of the gills (Fig. 21 

 B, k\ which arise in the same way in Teredo. The branchial fold 

 is covered by fine cilia. Thus, in Cydus and Teredo, the earliest gill- 

 rudiment exhibits the form of a plate, and at first shows no indication 

 of the slits and bars so characteristic of the adult. The further 

 differentiation of the gills, resulting in the formation of the gill-slits 

 and filaments, takes place from the anterior end of the fold. Starting 

 from the lower free edge, a series of vertically placed parallel grooves 



