14 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



gill, separated from the blood-spaces between the muscular walls 

 of the suspensory membrane only by occasional strands of connective 

 tissue. This space communicates laterally with the cavity of each 

 gill plate. Mitsukuri 8 supposed that these blood channels in Nucula 

 were connected with some definite blood circulation. In Yoldia there 

 seems to be no evidence that the blood follows a very definite course. 

 The blood from the gills passes into the auricles, which are situated near 

 the anterior extremities of the gills. It seems probable that the move- 

 ments of the blood in all of the blood-spaces of the gills tend toward the 

 auricles. The continuous longitudinal spaces are probably the means 

 of equalizing pressure throughout the gills, by allowing a free move- 

 ment of the blood to different parts as circumstances demand. 



s Diagrammatic transverse section of 

 --m Yoldia, showing the position and at- 

 '" = ? tachment of the gills, f, foot; g, gill ; 

 gs, gill suspensory membrane ; m, man- 

 tle ; s, shell. 



It will be remembered that the gills are suspended by muscular 

 membranes (gs, in the accompanying diagram), which probably repre- 

 sent folds of the body wall. The gills, g, are composed of wide plates 

 that are sufficiently wide to span the space between the foot, f, and the 

 mantle, m, and, behind the foot, unitedly to span the mantle-chamber 

 itself. Anteriorly they gradually diminish in size and finally 

 disappear. Posteriorly they are attached to the wall that separates the 

 inhalent from the exhalent siphon. The mantle-chamber is thus 

 divided by a movable partition, into a ventral chamber, opening 

 through the inhalent siphon and a dorsal chamber, opening through 

 the exhalent siphon. 



In young specimens the movements of the brown gills are visible 

 through the shell. They are gradually pressed ventrally, probably by 

 the blood forced into them, possibly aided by the cilia, which, 



