MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. I4l 



Thouo'h there is rarely any considerable diii'erence in the size of the gills, it will usually be 

 found that the gills of the anterior pair are thicker than those of the posterior pair, while the 

 latter may be a trifle the longest. Average measurements for the anterior gills are 18 millimeters 

 in width and 11 millimeters in thickness. The posterior gills are 15 millimeters in width and 9 

 millimeters in thickness. Both pairs of gills are about 5 centimeters in length. Each gill is 

 composed of two rows of flat, crescentic leaflets attached alternately to the sides of an oval stem. 

 Although the posterior gills possess somewhat the smallest bulks, they bear more leaflets than the 

 anterior gills, having about sixty-five pairs, while the latter have only about fifty-five. Owing 

 to the arrangement and shape of the leaflets the gills are flattened dorso-ventrally. The number 

 of leaflets varies considerably in difl'erent specimens, being as low as thirty in some. The 

 leaflets are attached upon the stem obliquely to the base of the gill. The leaflets of the tip of 

 the gill are rudimentary; they become more complex towai'd the base of the gill, being com- 

 pletely formed at about the tenth from the tip. 



It will be noticed that in this description I have apparently reversed the position of the gills 

 as given by previous observers. This is explained 1iy the fact that others have described the 

 gills in the position they occupj' after the mantle has been reverted, and as though they were 

 attached to the body wall. Owen recognized the pallial position of the gills, but later observers, 

 with the exception of Joubin, seem to have assumed that the gills are upon the body wall, and 

 this makes the posterior and anterior gills of their descriptions correspond to the anterior and 

 posterior gills of my description. 



The stems of the gills are flattened at their liases in the plane of the greatest width of the 

 gills to form thin plates. (Fig. i, at the end of the index line B V.) The lines of attachment of 

 the stems of the gills to the inner side of the mantle are directed upward, foi-ward, and slightly 

 outward. The anterior gills lie a little to the outside of the posterior gills. The branchial vein 

 runs along the ventral side of each gill as a projecting ridge. The In-anchial veins of the anterior 

 gills run inward from the bases of the gills toward the median line, suspended by a thin liga- 

 mentous band, which may be considered as an inward extension of the base of the stem of the 

 gills. (Fig. 1, B V.) At the outer borders of the inner renal sacs the veins pass inward through 

 the mantle toward the heart. The lower leaflets of the anterior gills extend inward for some 

 distance upon the ligamentous support of the branchial veins. 



Each leaflet is composed of two parts, a central or basal supporting portion of the same 

 tissues as the stem of the gill and an outer folded respiratory^ portion. (Fig. 23.) A branch of 

 the branchial vein passes along the outer edge of each leaflet. Between the smooth liasal part 

 of the leaflet and the collecting vein the leaflet appears to be ridged. In n^ality it is not ridged, 

 but folded or tucked, and the folds of the tucks alternating upon one side with those of the other 

 side form the apparent ridges. The sides of the folds, however, have fine folds nearly at right 

 angles to the edges of the primary folds. Along both edges of the folds are small blood vessels 

 running at right angles to the marginal vessels of the leaflet. 



The branchial vein lies upon the surface of the gill, forming a projecting ridge. Above the 

 branchial vein the leaflets of the opposite sides of the gill are united to each other in such a way 

 as to form a longitudinal septum extending between the vein and the stem of the gill. There is 

 no branchial canal in the gill of Nautilus. Upon the dorsal side the leaflets project considerably 

 beyond the stem of the gill, which is thus hidden in a groove between them. The stem is con- 

 siderably wider upon its dorsal than upon its ventral edge, and thus the leaflets come to be quite 

 widely separated dorsally. As the tip of the gill is approached the stem narrows. Toward the 

 base of the gill the dorsal side of the stem is produced into a sharp ridge. From the distal end 

 of the ridge to the tip of the gill the stem is marked by transverse grooves, extending from side 

 to side between successive pairs of leaflets. 



The branchial arteries pass outward from the vena cava through the posterior walls of the 

 nephridial chambers. Here they form outpocketings into the glandular appendages upon both 

 sides of the walls. From here the arteries pass into the mantle and run in it along the bases of 

 the gills until they reach the middle of the attachment of the gills to the body. At this point 

 they turn and run along the ventral side of the stem of the gill to its tip. I have never been 



