No. 480] 



BLOOD GILLS OF SIMULIUM 



883 



•central gill bases. From thence it passes dorso-cephalad and 

 laterad until it joins muscle a. Muscle c, a smaller, yet plainly 

 marked band of about 12 /i diameter, is inserted in the ijill at the 

 same point as b, and extends ventro-caudad and slightly laterad 

 until it reaches the floor of the abdomen where it attaclitvs to an 

 infolding of the body wall. This infolding extends traiis\ tersely 

 across the abdominal floor just <'e])halad of the catidal sucker. 

 Muscle d, the diameter of whicli is sli>iiit]y less than that of c, is 

 inserted just mesad of c and extends ventro-caudad and niesad 

 until it fuses with muscle c' of the right side. 



As the muscles of the right side correspond to those of the left 

 they require no separate discussion. 



When the gills are pushed out, the muscles are drawn with the 

 base into the everted part of the rectum, and when they contract 

 the gills are pulled back to their place inside. 



The presence of functional tracheje in their cavities points to the 

 •conclusion that these organs must, in some degree, function as 

 tracheal gills; for, if they act only as blood gills, the epithelial cells, 

 inasmuch as the oxygen before reaching the blood must pass 

 through their substance, would need no tracheation. But the 

 <'omparatively small number and size of the tracheie supplied to 

 the gills, together with the undoubted presence of blood in their 

 ■cavities, render untenable the hypothesis that they function wholly 

 as tracheal gills. 



In the light of this work it is evident that, except in a remote 

 morphological sense, not all gills are extensions of the body wall 

 but that some are borne by the reetuni. and it is also evident that 

 not all insect gills act wholly either as tracheal or as blood gills, but 

 that some function as both. 



In conclusion, I wish to a.-kn..\vledge niv indebiednos to Pro- 

 fessor J. II. (onistock in who>e laboratorv tin. research wa. 

 prosecuted, and to Dr. William A. WWvy who-,' constant and kindly 

 <Titici>m has l.een invalual.le. 



