MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 105 



GENERAL SKETCH OF ANATOMY. 



The Nautilus occupies a light gracefully formed shell, which is beautifully marked with 

 alternating bands of reddish brown and white, except near the mouth, where the dark bands do 

 not interrupt the white ground. (Fig-. 11). The shell is coiled dorsally (exogastrically) so that the 

 older parts are completely hidden by the younger. An adult shell has about two and a half 

 whorls. As the Nautilus increases in size it grows forward in its shell, leaving an unoccupied 

 space behind itself. At frequent intervals it forms septa which completely separate the unoccu- 

 pied portion from the living chamber of the shell, except at the centers of the septa, where they 

 inclose a tubular process of the body wall, which extends to the oldest chamber of the shell. 

 (Fig. 1.) The unoccupied chambers of the shell are filled with a gas which resembles air in its 

 composition, but having a slightly greater proportion of nitrogen than the latter. 



When the Nautilus is in its natural position the involution of the shell is uppermost, while the 

 mouth of the shell is below and turned forward. (Fig. 1.) The body is now nearly horizontal; 

 the head, surrounded by tentacles, is anterior; the siphuncle is near the posterior end of the 

 body; the funnel is upon the ventral side, and the depression which receives the involution of 

 the shell marks the dorsal side of the body. (Fig. 2.) The orientation of the body and the 

 description of the anatomy are made more difficult by the upward curve of the posterior part of 

 the body. Thus, the siphuncle springs from the dorsal surface near the posterior end. and not 

 from the extreme posterior end of the body. (Fig. 2.) 



The body of Nautilus is roughly oblong, and between »> and 7 inches in length. The anterior 

 portion of the animal which projects from the shell, comprising the cephalic and nuchal legions. 

 with the jaws, organs of sense, motion, and adhesion, is tough and muscular. The posterior 

 portion of the body is covered and protected by the shell, and consequently the body wall is here 

 so thin that the viscera can easily be distinguished through it. 



The part of the body contained within the shell conforms exactly to the latter. In the middle 

 dorsal region the body is deeply hollowed to receive the involution of the shell. All other 

 portions of the body within the shell are smoothly and evenly convex. The anterior part of the 

 body is covered by the large triangular hood. At the sides and ventrally are the numerous 

 digital tentacles, the sheaths of which, fused to each other and the hood, form a solid wall around 

 the mouth parts, to which the name cephalic sheath has been given. Within the cephalic sheath 

 are the projecting parrot-like mandibles, surrounded by several groups of smaller tentacles. 

 Beneath is the large muscular funnel, the crura of which pass upward upon the sides of the body 

 just back of the bases of the tentacles to the upper edges of the body, where they are joined by 

 the crescentic ridge extending across the back of the hood.. (Fig. 2, CR.) In the angle between 

 the projecting posterior corner^ of the hood, the back of the cephalic sheath and the crura of the 

 funnel, are the large round eyes (F). Two tentacles, the pre- and post-oculars (Fig. 1, O'. O"), 

 spring from close to the base of each eye, while just beneath each eye is a small projecting 

 pyramidal organ supposed to have an olfactory function. 



The mantle fold projects freely entirely around the body. The edge of the mantle is attached 

 along the edge of the shell, but from the umbilicus of the shell upw T ard it extends as a convex 

 fold covering the dark portion of the involution of the shell. The mantle cavity is shallow dor- 

 sally, while very deep and capacious ventrally. The organs of the pallia! complex are all within 

 the ventral portion of the mantle cavity. (Fig. 3.) The four gills, the renal, pericardial, and anal 

 orifices, and certain papillae sometimes spoken of as osphradia, are borne by the mantle. In adult 

 females the inner side of the mantle also bears a large bilateral nidamental gland. (Fig. 4, N.) 

 The reproductive orifices are situated upon the body wall near the origin of the mantle. The 

 mantle is continuous posteriorly with the thin and transparent wall of the visceral portion of 

 the body. 



The body wall is produced from the end of the body as a slender tube, the siphuncle. which 

 extends within the siphon of the shell to its very end. (Fig. 1, fig. 2.) At the sides and above 

 the middle of the body are seen the crescentic areas of attachment of the strong shell muscles by 

 which the body is held to the shell. (Fig. 1. SM.) Beside these areas of attachment, the wall of 



