MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES, 127 



A longitudinal section of the cirrus shows that the central transverse musculature consists of 

 alternating layers of crossing fibres. (Fig. 52.) The section figured in Fig. 53 has been cut a trifle 

 obliquely so that it passes through several of these alternating layers. The layers of muscles are 

 arranged like the crossing boards of a double floor. 



The surface of the cirrus is covered by a tine columnar epithelium in which are scattered 

 great numbers of goblet cells. The epithelial surface is increased by numerous pits the lining 

 epithelium of which is especially supplied with secreting cells. 



The nerve of the cirrus is enlarged at regular and frequent intervals, like the nerves of the 

 digital cirri. Each enlargement corresponds in position to a pair of the alternating layers of the 

 transverse musculature. In a young Nautilus the first cirrus of the as-yet-undeveloped spadix is 

 like one of the labial cirri, slender and marked with annular grooves. The corresponding seg- 

 mentation of the cirrus and its nerve in the digital and labial tentacles has already been noticed. 

 Probably there is a similar correspondence here, while the transverse musculature is also seg- 

 mentally arranged. As the cirrus under discussion increases in size the external segmentation 

 becomes obliterated. The branches of the nerve pass outward in the layers of the transverse 

 muscle. The nerve ends abruptly near the tip of the cirrus. An artery runs along the inner 

 side of the nerve A. The vein divides and its branches come to lie at a considerable distance 

 from the nerve, V, V, Fig. 53. 



Between the outer layer of muscles and the epithelium is a layer of connective tissue which 

 is curiously developed in one region. (Fig. 52.) Over most of the cirrus the layer is thin and the 

 tissue firm and close, containing a few muscle fibres; but just below the tip it is much thickened 

 and great numbers of vascular lacuna;, large and small, make their appearance in it. The larger 

 lacuna? have endothelial walls. Fig. 52 only represents the larger lacuna?, and not the far greater 

 number of small ones. It may be possible that this forms a kind of erectile tissue. 



SECOND CIRRUS OF SPADIX. (FIG. 16.) 



The second cirrus of the spadix is much more modified than the first, for that is modified in 

 shape and size mainly, while this has undergone modification of structure also. It is slender, and 

 instead of becoming larger at its base it narrows. Its muscles are continued into the sheath of the 

 spadix between the first and the third cirri. (Text-fig. 5, C, 2.) Its length is about 4 centimeters. 

 Its basal portion, to within about 15 millimeters of the tip, is round and smooth. Exceedingly 

 indistinct annular grooves can sometimes be seen in this portion of the cirrus. At the point 

 referred to the cirrus begins to be flattened upon both dorsal and ventral surfaces. It ends in a 

 flat, thin-edged, lancet-like tip. A little distance from the tip one edge remains thick while the 

 other is thin, giving the cirrus a triangular shape. Concurrently with the flattening the cirrus 

 becomes more and more distinctly annulated, the grooves appearing a little more strongly marked 

 upon the ventral than upon the dorsal surface. 



In general this cirrus adheres to the plan of structure already described for other cirri. The 

 nerve, extending through the cirrus near its center, does not possess any unusual characters. It 

 is of good size, showing ganglionic enlargements at regular intervals which correspond with the 

 annulations upon the surface of the cirrus, and extends to the very tip of the cirrus. (Fig. 61, N.) 

 Upon the inner side of the nerve is an artery, and still farther toward the inner edge of the cirrus 

 a large vein. (Fig. 56, A and V.) The nerve and the artery are surrounded by transverse muscle 

 fibres, but the radial arrangement of the musculature has been lost to a large extent and is 

 replaced by an arrangement of the longitudinal muscles in lines extending across the cirrus in the 

 direction of the shorter transverse axis, the dorso-ventral axis. In the round basal portion of 

 the cirrus the arrangement of the muscles is similar to that in the digital cirri. 



The dorsal side of the cirrus, amounting to from one-half to one-third of its entire, thickness, 

 from the point where the annulations first show plainly, is occupied by glands and not by muscles. 

 (Figs. 56 and 61, G.) The glands open by very minute pores in the annular grooves, a single 

 row of closely placed pores occupying each groove. The pores of the glands are upon the slopes 

 of the grooves and not in the bottoms. They are so small that they are only to be seen in sections. 



