s 



150 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the fuunel is, to all intents and purposes, a tightly closed tube. Apparently it makes no differ- 

 ence which edge is outermost, though in most cases the right side of the funnel overlaps the left. 

 There is no structural difference between the two sides. 



The crescentic ridge running in an almost horizontal line around the concave posterior side 

 of the hood is so closely related anatomically to the crura as to almost seem to be a continuation 

 of them. Its similar shape and relation to the surface of the body point to a similarity of func- 

 tion, at the least. The ends of the crescentic ridge arise upon the dorsal or inner sides of the end,> 

 of the crura (Figs. .5 and 7); these surfaces correspond to the anterior surfai'cs of the lower parts 

 of the crura. In this way the crescentic ridge unites the upper ends of the crura. 



Aside from the great mass of muscle forming the posterior portion of the funnel and its crura, 

 which can not be separated into separate muscles, the funnel possesses two distinct and independent 

 nmscles. These (the Leoatores infundilidl Owen) pass from the central part of the cephalic 

 cartilage outward and forward in the lateral parts of the roof of the funnel to near its tip. They 

 are slender round muscles, '2 millimeters in diameter and Zh millimeters in length. They lie in 

 smooth-walled cavities or tunnels in the tissue of the funnel, so that they are attached only at 

 the two ends. The anterior ends spread out in the tissues of the roof of the funnel a few milli- 

 meters in front of the base of its valve. They seem to have no connection with the valve. On 

 account of their being attached at the ends only, the contraction of these muscles probably affects 

 only the tip of the funnel, pulling or bending it upward. The tip would be straightened or bent 

 downward by the intrinsic nuiscles of this part of the funnel. The length of the levatores must 

 allow a considerable contraction, and consequently they are able to cause a considerable motion of 

 the tip of the funnel. Their position and course is indicated externally by slight ridges upon the 

 surface of the funnel where it touches the edges of the ventral groove of the cephalic sheath. 

 Blood lacunte open into the sheaths of the levatores. 



The funnel receives its blood through two arteries which spring from the tentacular arteries 

 near their origins, and which, according to Willey (1896, p. 178), "pass through the cartilage 

 into the funnel." (Text-lig. 10, p. 182.) 



The infundibular nerves are two in number, each of considerable size. They are the inner- 

 most nerves springing from each pedal ganglion. (Fig. 11, 8.) 



VISCERAL BODY WALL. 



The body wall of the posterior end of the body of Nautilus is very thin, soft, and delicate. 

 In some cases semitransparent, the outlines of several of the viscera can be distinguished through 

 it. The greater ])art of this portion of the body wall is applied to the last septum of the shell, 

 and the septa are formed by its epithelium. The thin Ijody wall extends out upon the outer side 

 of the mantle fold, so that there is no external sign of the posterior limit of the mantle fold. 

 The thin body wall of the visceral region is composed of fibrous tissue in which are numerous 

 tine branching l)lood ves.sels and a very noticeable nervous plexus. P^xternally it is covered by 

 a layer of short columnar epithelial cells. Internally — that is, upon the coelomic side — it is 

 covered by comparatively large Hat polygonal pavement cells. At the posterior end of the body 

 (dorsally as the animal is curved) the body wall is produced into a long slender tube, the 

 siphuncle, which passes through the siphon to the last chamber of the shell, where it probably 

 ends blindly, like the siphon. The body wall is firmly attached to the shell over the end of each 

 shell muscle and along three aponeurotic bands extending Ijetween the ends of the attachments 

 of the shell muscles. (Text fig. 7.) Everywhere over these aponeurotic areas, a thin plate or 

 ribbon of chitinous mateiial is found between the body wall and the shell. This is secreted 

 by the epithelial cells of the aponeurotic areas and seems to form a kind of cementing substance 

 by means of which the body wall is firmly attached to the smooth surface of the shell. 



The attachments of the body wall to the shell over the ends of the shell nuiscles form areas 

 of the same shape as the ends of the muscles. (Fig. 1, S M.) They are roughlv crescentic, or 

 perhaps better, approach in shape spherical right-angled triangles, the right angles being 

 directed forward and downward. 



