112 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Iirown pii;inent. In .sections the pigment appears as a dark l)and 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 

 In-ai' Muniereus small pits lined with the pigmented epithelium. Granular cells are exceedingly 

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

 dark brown, sometimes black, layei- 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 layei'. 



The great density of the elastic tissue makes the cephalii^ sheath exceedingly tirni and ditii- 

 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. 4'.t.) 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 imier 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 til)res surrounding the nerve are mostly trans- 

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

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

 radial arrangement is almost lost upon the inner side of th(> 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 band of oblique muscles (Fig. 51, OM). 

 Outside this is a layer of small bundles of longitudinal nuiscle flbres (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 ti'ansverse 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, but 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 nuiscle fibre. The fasciculi of the longitu- 

 dinal nmscles 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 fi'om aljout the nerve, passing between 

 the longitudinal nmscles 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 nmscles 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 nt)t disturbed. After repeated examinations I am convinced that these are actually mus- 

 cle fibres and not some form of connective tissue which, in the in\'ertebrates, 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 n to 600 //), 'Hid 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. 



Tlic retraction of tlic cirrus is accomplished V)V the longituiluial muscles; the elongation by 



