142 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



able to find the valves mentioned by Owen as existing in the branchial arteries at the bases of 

 the gills. The minute structure of the gills of Nautilus pompilim and mttcr<>ntj>h<ihiN has been 

 studied by Joubin. He considers that the thickened stem of the gill of Nautilus forms a 

 structure which is comparable with the glands he has described in the gills of other Cephalopoda, 

 which are organs in which the blood corpuscles are formed. 



The stem is covered externally by a layer of muscle fibres which also pass outward along the 

 concave edge of each leaflet. The stem is composed almost entirely of connective tissue, in 

 which the glandular elements are disposed at several points. 



Near the outer surface of the stem is a layer of quite large vascular lacuna?, incompletely 

 lined with flattened endothelial cells. The lacuna? are separated only by thin lamellae of con- 

 nective tissue. The lacuna? also extend outward into the supporting portion of each leaflet. 

 Only very small lacuna? are found in the center of the stem. In the middle of the outer side 

 of the stem a large vein is constantly present, extending from the tip of the stem to near its 

 base, where it opens into the branchial artery. 



In the central portion of the stem, but still near the median vein, are a number of lacuna* 

 which, instead of being empty, like those at the surface of the stem, are filled with large granu- 

 lar cells, each possessing a large nucleus. These may be found throughout the remainder of the 

 gland (or stem), scattered here and there in the connective tissue. Joubin /considers that the 

 central region is the only portion presenting an aspect comparable to that observed throughout 

 the whole of the glands of othe" Cephalopods. 



The remainder of the stem (gland, Joubin terms it) is formed of connective tissue, in which 

 large numbers of muscle fibres are scattered. 



The concave border of each leaflet is thickened, the thickening being due to a lateral extension 

 of the tissues of the stem of the gill into each leaflet. (Fig. 23.) It forms the supporting part of 

 the leaflet, as well as containing portions of the branchial gland. It contains very numerous 

 vascular lacuna?, which differ in their arrangement from those of the etern in that tin? larger 

 lacuna? are in the central portion, while the smaller lacuna? are external. The lacuna? receive 

 blood from the afferent vessel of the leaflet. .Here and there are free cells in the lacuna? similar 

 to those observed in the principal part of the gland. All this region of the leaflet is covered 

 with tall, columnar, epithelial cells. The outer ends of the cells bear a distinct border. 



The thin respiratory membrane is covered with large, flattened, epithelial cells. A great 

 number of lacunae penetrate the inner part of the membrane, scarcely separated by a network of 

 connective tissue cells. 



A band of muscle continued from the surface of the stem passes outward along the side of 

 the afferent vessel turned toward the respiratory membrane. In places the muscle projects into 

 the afferent vessel and is bathed by the blood flowing in it. 



The blood reaches the gill through the branchial artery which runs along the inner side of 

 the stem from the base to the tip of the gill. (Fig. 23, 2.) Two sets of vessels are given off from 

 the branchial artery. Upon opening the artery two rows of large alternating openings are seen 

 in the wall turned toward the leaflets. These lead into the afferent vessels of the leaflets, which 

 run along the concave side of each, close to the edge of the respiratory membrane. Two other 

 rows of alternating openings, smaller than and median to the first, lead into vessels which 

 pass into the stem of the gill. 



From the afferent vessels of the leaflets smaller vessels arise which pass at right angles to 

 the first along the entire length of the upper edges of the primary folds of the respiratory mem- 

 brane. Opposite each secondary fold is an opening in the wall of the vessel of the primary fold 

 through which the venous blood passes into the lacuna? of the secondary fold, where the inter- 

 change of gases between the blood and the water takes place. 



In a similar but inverse manner the arterial blood is collected from the secondary folds into 

 efferent vessels lying along the lower edge of each primary fold. These unite in a vein following 

 the outer or convex border of each leaflet, which vessels themselves unite with the branchial 

 vein. 



From the afferent vessels of each leaflet the blood escapes through several openings into the 



