140 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



of these muscles must be. The outer sides of the muscles are convex, the inner sides concave. 

 The shape of the muscles in cross section is about the same as the shape of the area of attach- 

 ment of the muscles to the shell. 



The muscles meet anteriorly, but as they pass outward and backward a considerable triangu- 

 lar space is left between their ventral edges. This portion of the body wall is composed of quite 

 a strong layer of transverse muscle fibres, which are continued, in part at least, outward over the 

 shell muscles. The actual thickness of this region of the body wall is very little, but it is 

 considerable when compared with that of the body wall covering the visceral region. The vena 

 cava lies in the middle of the triangular space between the shell muscles. 



Between the dorsal edges of the shell muscles is another thin portion of the body wall, 

 which forms the floor of the dorsal portion of the mantle cavity. This is also supplied with a 

 quite strong musculature. This dorsal region of the body wall is concave, like a mold of the 

 involution of the shell. It passes into the hood anteriorly and laterally, becoming gradually 

 thicker and firmer. 



The shell muscles have a very copious supply of blood received through a number of arteries. 



The posterior columellar arteries are the largest and most important of those supplying the 

 muscles. The left posterior columellar artery arises with the hepatic artery from a short common 

 branch of the dorsal aorta — the hepatico-columellar artery. (Text-fig. 10, p. 182.) The columellar 

 artery takes a diagonal course upward, outward, and forward to the upper edge of the left shell 

 muscle, immediately anterior to its attachment to the shell. Then, bending downward and back- 

 ward upon the inner surface of the muscle, it gives off numerous branches which penetrate the 

 muscle. The right posterior columellar muscle arises from the right side of the dorsal aorta 

 about 7 millimeters anterior to the origin of the hepatico-columellar muscle. As the posterior 

 portion of the dorsal aorta lies upon the left side of the body, the right posterior columellar artery 

 has a longer course to run to reach the right shell muscle than the left posterior columellar 

 artery has to reach the left shell muscle. Otherwise the course of the two arteries is the same 

 upon their respective sides. 



From the point where each posterior columellar artery bends downward a branch is given 

 off to the region of the mantle in front of the shell muscles and to the portion of the body wall 

 dorsal to it. 



The anterior portions of the shell muscles are supplied with blood by a branch of each 

 innominate artery. These (the anterior columellar arteries) pass backward, downward, and 

 outward to the muscles. (Text-fig. 10, p. 182.) 



In addition to these special arteries to the shell muscles, the nuchal arteries, or their 

 branches pass along the inner dorsal edges of the muscles and appear to send small branches 

 into them. 



The shell muscles are innervated from the visceral ganglia. Exceedingly numerous flattened 

 band-like nerves pass from the posterior side of each ganglion into the muscles. (Figs. 41, 1-1.) 



GrLLS. 



The two pairs of gills are situated upon the mantle at the sides of the ventral mantle chamber. 

 (Figs. 3 and 4). They are arranged as an anterior and a posterior pair, one gill of each pair lying 

 upon each side of the mantle, close to each other. The gills of Nautilus differ from those of the 

 Dibranchiata, not only in number, but also in that they are situated upon the mantle instead of 

 upon the body wall, and in that they are attached by their bases only, otherwise lying freely in the 

 mantle chamber. They have much the same shape and structure as the gills of the Dibranchiata. 

 The gills are situated upon the thin and muscular part of the mantle just below the convex anterior 

 angle of the outer end of the shell muscle, the base of the anterior gill lying about 8 millimeters in 

 front of the base of the posterior gill. The bases of the gills are sometimes placed so closely to 

 each other that they seem, as Owen said, "each pair arising b)' a common peduncle, from the 

 inner surface of the mantle."' though I have never found this to be literally true. Examination 

 always has shown that the bases of the gills were separate from each other. 



