52 



which (except the anterior ten) do not fonn I'ilaments. 

 From tlie mode of formation it is seen that there is a 

 large flat blood space in each la'nina, and that there is 

 a free flow of blood through the lamina (in the direction 

 of the arrows fig. 37) between the afferent and efferent 

 branchial veins. 



In a yovmg Teredo a half centimeter in length (some- 

 v/hat later thai: ..^ stage represented in Pir^, G), there is 

 on either side a continuous series of seventy-five or more 

 gill filaments (filaments and laminae), stretching from 

 the mouth region, around the sides of the body, and pos- 

 terior to the visceral mass. Soon afterwards the "fila- 

 ment" between the tenth and eleventh (usually) gill-slits 

 broadens from before back'.vards. This grovrth increases 

 till, in the adult, the anterior ten filaments (it may 

 rarely be nine or eleven) are separated from the rest of 

 the gill by ten cm. or more, in large specimens. However, 

 they retain the structure and, doubtless, the function of 

 gill elements, though in the adult, they form a series of 

 plications on the sides of the "head" (fig.9). 



In Teredo, the epibranchial cavity forms a long ca- 

 nal posteriorly (figs. 10,33,34 J but is divided anterior- 



