48 



f. The Respiratory System. 



The gills of Teredo are perhaps more highly special- 

 ized than those of any otiier type of Lainellibranch. For, 

 besides possessing a membranous, non-perforato portion 

 which reminds one of the gill structure in the Septibran- 

 chia, they are otherv^ise sharply marked off from those of 

 forms nearly related to the "ghip-worms". 



The Development and General Structure of the Gills.- The 

 embryonic development of the gills of Teredo has been ob- 

 served by Hatschek (6) in the viviparous larva of the un- 

 identified species studied by him. Here the rudimentary 

 gill of either side is a fold, in vrhich perforations ap- 

 pear in succession, new ones being added postei'ior to 

 those already formed. In the newly-attached larva, the 

 gills have advanced but little beyond the stage described 

 by Hatschek. On either side there are two slits and the 

 rudiment of a third. However, the slits have so increased 

 in size as to occupy most of the space on the upper sides 

 of the foot, and the gill-fold has fused to the sides of 

 the foot by its ventral edge. In this way, the triii-snts 



