209 



into a few tilanients, fdrniiiiL; tlu- anturidr gills (fig. 76, a.i^.) just back of the labial 

 palps. 



The free ventral edge of each gill is characterized by a longitudinal groove, the 

 branchial groove (fig. 76, li-g,-). which extends uninterruptedly from the posterior to 

 the anterior extremity of the gills. This gro()\e is lined with cells which bear cili.i, 

 the function of which is to s-weep food particles forward to the region of the mouth. 



The structure which connects the anterior gill with the posterior gill on each side 

 consists mereh' of a rudiment suspended from the roof of the infrabranchial cavitN', 

 which bears along its ventral margin the liranchial groove. This rudiment of the gill 

 is not divided into transverse filaments, but itself represents morphologically a single 

 greatly broadened filament (Sigerfoos, l')()8). 



au, s pa d I ag sh (J I p a ( 



Fig. 76. Teredo iiavalis, external view of the anterior end of the bod\' from the right, x 7. Part of 

 the right valve and mantle have been dissected away. a.a. Anterior adductor muscle, a.f. Anterior 

 pallial fold. a.g. Anterior gills, aii.s. Auricle of the shell, b.g. Branchial groove, d.f. Dorsal pallial 

 fold, rf./.p. Dorsal labial palp./. Foot. rt. f. Infrabranchial cavity, m. Mouth, ma. Mantle, p.a. Posterior 

 adductor muscle, sh. Shell, v. a. Ventral articulation of the shell, v.l.p. Ventral labial palp. 



When Teredo is undisturbed, the respiratory process is carried on continuously. 

 A stream of water, drawn in through the incurrent siphon, passes forward in the 

 infrabranchial cavity, over the gills and through the gills, between the transver.se 

 filaments, into the suprabranchial cavity, whence it passes again to the outside of 

 the body through the excurrent siphon. Thus there is normally a continual movement 

 of water forward in the infrabranchial and backward in the suprabranchial cavity, 

 while through each slit between the transverse filaments a small portion of the moving 

 water passes from the incoming to the outgoing stream. The interior of each lamina 

 of the gills is occupied by blood spaces, and as the water passes between the laminae, 

 oxygen is absorbed from the water and carbon dioxide is given off. 



The oxygen requirements of lamellibranchs are extremely low, and it is probable 

 that the respiratory process can be suspended for an indefinite period of time without 

 serious inconvenience when the animal is disturbed, or is forced by unfavorable 

 conditions to retract its siphons and close the burrow with the pallets. In such in- 

 stances the water retained in the burrow alTords a supph' of oxygen for a time, and the 

 oxygen requirement of the occupant is still further reduced by the cessation of most 

 of its normal activities. Even so, the teredo will often survi\e in a closed burrow long 

 after the oxygen supply of the water in the burrow is presumably exhausted. 



