112 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



brown pigment. In sections the pigment appears as a dark band near the outer ends of the cells, 

 a narrow unpigmented band being outside this, while the pigment granules gradually disappear 

 toward the bases of the cells, leaving this region also uncolored. These surfaces of the hood 

 hoar numereus small pits lined with the pigmented epithelium. Granular cells are exceedingly 

 numerous, especially upon the concave posterior face of the hood. It seems probable that the 

 dark brown, sometimes black, layer of organic matter found upon the involution of the shell is 

 deposited by these cells. The thin crescentic ridge which projects from the posterior face of 

 the hood does not possess a pigmented epithelium and may not play any prominent part in the 

 deposition of the dark layer. 



The great density of the elastic tissue makes the cephalic sheath exceedingly firm and diffi- 

 cult to penetrate. It must afford a very considerable protection to the Nautilus. 



A cross section of a cirrus shows a highly muscular organ, usually of triangular outline 

 in its terminal third. (Fig. 49. ) Near the center is a large nerve which extends the entire length 

 of the cirrus. (Fig. 50, N.) The nerve is situated toward the inner side of the cirrus, i. e., toward 

 the flattened side turned toward the mouth of the animal. Close to the nerve and upon its inner 

 side is a small strong walled artery (A), and still closer to the inner side of the cirrus is a some- 

 what larger vein (V). The nerve is surrounded by a sheath of connective tissue and muscle 

 fibres (T), while outside this is a mass of longitudinal muscles forming the greater part of the 

 cirrus (LM). The connective tissue and muscle fibres surrounding the nerve are mostly trans- 

 verse to the axis of the cirrus, and they pass outward in such a way as to divide the longitudinal 

 muscles into radiating bundles which appear in cross section like the spokes of a wheel. The 

 radial arrangement is almost lost upon the inner side of the nerve, where the muscle bundles 

 become small and irregularly arranged. The radial longitudinal muscles do not extend to the 

 periphery of the cirrus. They are bounded by a narrow hand of oblique muscles (Fig. 51, OM). 

 Outside this is a layer of small bundles of longitudinal muscle fibres (LM.); while outside the 

 latter is a thin layer of circular muscle fibres (CM). The outer muscular layers lose their identity 

 upon the inner side of the cirrus, where transverse muscles predominate. A thin layer of con- 

 nective tissue frequently separates the external circular muscles and the epithelium. The peculiar 

 arrangement of the longitudinal muscles persists for a time after they enter the body wall at the 

 base of the cirrus, hut it is then lost as the muscles separate. 



If, now, we examine the radially arranged longitudinal muscles more carefully' we find that 

 each muscle is composed of a large number of small fasciculi, held in a mesh work of connective 

 tissue into which penetrates an occasional transverse muscle fibre. The fasciculi of the longitu- 

 dinal muscles do not extend straight up and down the cirrus, but have a slightly oblique course 

 upward and inward; i. e., as the muscles pass toward the tip of the cirrus the fasciculi pass from 

 the outer to the inner side of the muscle. (Fig. 50, LM.) The fibres of the fasciculi, however, 

 take a course parallel to the axis of the cirrus and are only rarely oblique to it. 



The transverse muscle fibres radiate in all directions from about the nerve, passing between 

 the longitudinal muscles and penetrating the outer circular layer. (Fig. 51.) The fibres are gath- 

 ered into strands, few of which, however, radiate directly outward from the nerve, though at 

 first sight they may seem to do so. Most pass in an hyperbolic curve from between two longi- 

 tudinal muscles to between two others about 90 degrees away- Under this arrangement, when 

 the transverse muscles contract, the outer portions of the cirrus alone are compressed and the 

 nerve is not disturbed. After repeated examinations I am convinced that these are actually mus- 

 cle fibres and not some form of connective tissue which, in the invertebrates, is often so hard to 

 distinguish from muscle tissue. There is also considerable connective tissue in the transverse 

 strands of the cirrus. 



The fibres of the longitudinal muscles of the cirri are unstriated, smooth, slender, and 

 exceedingly long (400 /< to ti00 /<), and tapering gradually to pointed ends. An oval nucleus 

 lies at the side of the cell near its middle. There is very little elastic connective tissue in the 

 cirrus. 



The retraction of the cirrus is accomplished by the longitudinal muscle-: the elongation by 



