50 



ternal scries internal to the ten first formed in the ex- 

 ternal, and none ever appear. 



The f^ill-fold and ciH ^^ the young Teredo represent 

 the internal half of the I.'olluscan ctenidium. From tlie 

 resemblance of the mode of development to tliat in Cyclas 

 (Ziegler) and *'ytilus (Lacaze-Duthiers ) , it is seen that 

 the slits of the first formed series separate the descend- 

 ing limbs of the Lame lli'o ranch ^:ill filaments; and that 

 the second series separate the ascending limbs. The ante- 

 rior* ten filaments, then, never develop the ascending 

 limbs. Likewise, the other half of the ctenidium is never 

 developed in Teredo, contrary to the belief of Deshayes 

 and Quatref ages , ViTho believed the whole ctenidium, or 

 "pair" of gills to be present on either side of the body. 



The term "gill-fold" I have used to designate the 

 posterior end or growing point of the gill, and "gill-fil- 

 aments", the elements that are formed from it. However, 

 in later stages, soon after that shown in fig. 3, the gi*ow- 

 ing point forms a more or less cylindrical hollow tube 

 filled by a blood space, v/hich fuses continuously on the 

 midline with its fellow of the opposite side, and dorsally 

 over a v/ide area (between th.e tv/o points indicated fus 

 in fig. 37) viith the mantle. In thir, vrny the ot ilrrnnrhial 



