51 



cavity is sepax'ated from the rest or the mantle cavity. 

 Meanwhile, the free portion oj' th.e j-irowing point has be- 

 come angular, and at tlie sides of the angle the two series 

 of perforations are fomied progressively (fig,6), the ex- 

 ternal alvmys slightly in advance of the internal. The 

 corros]'onding slits of the tv/o series push in (in the di- 

 rection of the lower arrows in fig. 37) till they meet 

 each other and till they push through to the epibranchial 

 cavity. These inpushings divide the original blood space 

 of the grov/ing point into narrow spaces separated from 

 each other except at two points .the openings into the af- 

 ferent and efferent branchial veins. The median portion 

 of the original blood space remains undivided as the affer- 

 ent vein, and by the disappearance of the middle part of 

 the walls of the tv;o growing points as tliey fuse together, 

 the afferent veins of the two sides imite to form the 

 large afferent branchial vein. The undivided dorsal por- 

 tion persists as the efferent branchial vein on either 

 side. The walls of adjacent slits are coniiected together 

 by nunnerous connective-tissue cells (fig. 33) , so as to 

 form the gill lar.iinacj , the name given to them by Quatre- 

 fegejs , and more appropriate for the gill elei.icnts in Teredo 



