154 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



posterior to the vestibule, while the intestine passes from its right side. (Fig. 27, I 1 , F, F.) Com- 

 municating with the intestine near its beginning is the caecum (Fig. 27, Coe), into which the duct 

 of the liver opens. The large lobes of the liver lie under and partially surround the alimentary 

 canal. (Fig. 27, L, L 1 .) The intestine passes around the posterior and right sides of the ciecum, 

 then turns downward and backward beneath the csecum and the stomach. It turns forward 

 again and passes above the heart to the anus. The branches of this loop run parallely, close 

 beside each other, and are connected by a mesentery. The terminal portion of the gut is slightly 

 constricted and is more muscular than the preceding parts, and has been termed the rectum. 



Since the jaws serve as points of attachment for the fleshy portions of the buccal mass, it is 

 necessary to describe their form before describing the soft parts. 



The jaws of Nautilus are like those of other Cephalopoda, in being large and hooked (much 

 like the beak of a parrot), in the upper fitting inside the lower jaw, in being formed for the most 

 part of chitin, and in each jaw dividing into two flanges a little distance back of the cutting edge. 

 (Figs. 30 and 31.) Unlike other cephalopod jaws, those of the Nautilus are coated with a hard 

 calcareous deposit for some distance back from the biting edges. Nor are they so sharply pointed 

 as the jaws of other cephalopods. The calcareous deposit thickens toward the edge of the 

 jaw, so that this, instead of being sharp, is broad and flat except for a few irregularities of the 

 surface. The. heaviest deposit is usually upon the upper jaw, the entire point of which is calca- 

 reous. Aside from this deposit the jaws are of hard, black chitin, extremely strong and light. 

 In Figs. 30 and 31 the two jaws are shown; the lower jaw has been split through the middle to 

 show its inner flange. 



The lower jaw is 30 millimeters in length by 28 millimeters in width; the upper jaw is 32 

 millimeters in length by 17 millimeters in width. The flanges of both jaws are unequal in size, 

 the inner flange of the lower and the outer flange of the upper jaw being much smaller than the 

 other flange of each jaw. It is as though the edge of the lower jaw had been turned inward 

 while the edge of the upper jaw had been turned outward upon itself. The larger flanges of each 

 jaw extend nearly to the back of the buccal mass, that of the lower jaw upon the outside, while 

 that of the upper jaw is deeply buried under muscles and forms the roof and sides of the 

 mouth. The smaller flanges of the jaws are broadest anteriorly, gradually narrowing as they 

 pass to the sides. 



The chitinous portion of the lower jaw is pointed in front, the contour of the jaw being 

 unchanged after the calcareous matter has been removed. The chitinous part of the upper jaw, 

 on the other hand, is blunt or even notched anteriorly, the sharp point of the jaw being entirely 

 formed by the calcareous deposit. 



The buccal mass projects from the body in the midst of the tentacles. The enormous jaws 

 with their muscles form the bulk of the organ; within is a comparatively small cavity well tilled 

 by the tongue and several projecting fleshy folds. Back of the cutting edges each jaw divides 

 into two thin flanges, an inner and an outer, to which the jaw muscles are attached. The external 

 flanges lie upon the surface of the buccal mass and may be seen by merely turning back the 

 buccal membrane. The internal flanges lie deep and are not seen until the buccal mass is nearlv 

 dissected. 



The skin of the head is carried forward upon the buccal mass, forming a mantle or collar 

 around the jaws, the buccal membrane. (Fig. 28, BM.) Upon and just within the edge of the 

 buccal membrane are a large number of more or less slender papillae. Those upon the edge are 

 frequently quite like small tentacles. The buccal membrane is attached to the buccal mass along 

 the posterior edges of the external flanges of the upper and lower jaws. It is readily seen, there- 

 fore, that the anterior portion of the membrane is in reality a double fold. Except near the edge 

 of the fold the outer and inner portions are actually separated by a space which is part of the 

 hamioccel. Near the edge the two parts are tightly bound together by muscle fibres and con- 

 nective tissue. The inner part of the fold — i. e.,that part which is attached to the jaws and 

 immediately surrounds them— is extremely thin and delicate except along the anterior edge. 

 Between the jaws and the buccal membrane is a space which, at the sides, where the external 



