MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 143 



lacuna' of the supporting portion of the leaflet. These are in communication with the lacunae of 

 the stem of the gill, and so the blood passes from the leaflets into the stem. From the lacunas 

 of the stem it is gathered into the median longitudinal vessel lying near the surface of the stem. 

 During its passage through the stem and its lateral extensions the blood has caught up many of 

 the free cells of the branchial glands, which now form new blood corpuscles. They are carried 

 by the longitudinal vessel into the branchial artery at the base of the gill, and thus into the circu- 

 lation. It will be noticed that all the blood passing through the stem of the gill is venous, and 

 that, having made this short circuit, it passes again into the branchial vein, the greater part then 

 passing through the branchial leaflets into the systemic circulation. 



An observation made by Willky on a young Nautilus is significant. Describing it. Willey 

 says: 



"The youngest individual I have as yet obtained was a male with the following dimensions: 



Millimeters. 



Length from root of siphuncle to mid-anterior point of hood i measured along the dorsum) 25 



Length of hood in middle line 10. 5 



Breadth of body across middle of eyes , 15 



"The surface of the hood was perfectly white and unpigmented. The branchiae of opposite 

 sides were in close apposition in the median line, and. curiously enough, the larger posterior pair 

 extended forward far into the interior of the funnel. 



"The shell was perforated at the umbilicus, as it is throughout life in X. innhilirniiix." 



If this specimen was a typical one of the young Nautilus it is evident that the gills are moved 

 outward toward the sides of the body as the animal approaches maturity, a fact which possesses 

 still more interest when we remember that the rudiments of the gills of the Dibranchiata arise 

 close to the median line of the body, on either side of the anus, and that they move to the sides of 

 the body late in development only. 



The branchial nerves are two large, flattened, band-like nerves, one of which arises from the 

 posterior side and near the inner end of each visceral ganglion. (Fig. -±1, 22.) They run directly 

 backward, along the ventral body wall, to the posterior limit of the mantle cavity. Here they 

 turn outward and forward in the inner wall of the mantle fold. After giving off a couple of 

 small branches which apparently supply the walls of the renal sacs, the nerves fork near the bases 

 of the gills, and a branch passes into each gill. (Fig. 41, 19 and 20.) 



The later shifting of the gills to the sides of the body may account for the peculiarly exposed 

 course of the branchial veins of the anterior gills. (Fig. 4, BV.) 



INTERBRANCHIAL AND PREANAL PAPILLAE. 



Just in front of the base of each posterior gill is a small papilla upon the inner surface of 

 the mantle. (Fig. 3, IP.) The papillae are about 2 millimeters in height and width. It is to 

 these papillae, situated between the bases of the anterior and posterior gills, that I apply the 

 name interbranchial papillae. 



In the median line of the ventral part, just in front of the line limiting the thin portion of 

 the mantle, two papillae project from the inner surface of the mantle. (Figs. 3 and 4, PA.) 

 Each papilla has the shape of a bilobed transverse ridge, and usually the two are fused, 

 forming a distinct ridge across the median line of the mantle. It is only rarely that the 

 papillae are separate, and even then the separation is so slight as to be scarcely noticeable. 

 Willey describes a case in which the interval between the papilla' was 2.5 millimeters. The 

 sometimes total separation and nearly constant partial separation of the two parts of the ridge 

 leads me to describe it as two papillae fused rather than as a single papilla. 



The united papilla' have usually passed, heretofore, by the name of postanal or supra-anal 

 papillae. As it is readily demonstrated that they are situated upon the inner surface of the 

 mantle and not upon the body wall, they can not be postanal except when the mantle is turned 

 back and the natural position of the parts of the pallia] complex is reversed. To avoid this 

 difficulty I suggest that they be called the preanal papilla?. 



In 1883 Lankester and Bourne first called particular attention to the interbranchial 



