MEMOIHS OF THE NATIONAL ACADEMY OF SCIENCES. 153 



If the crura should now Ix^ swopt inward like fans, touching the inner surface of the mantle 

 as long as is possi))lc. the volume of th(> mantle cavity would be slightly reducetl and some of its 

 contained water would he expelled. If this fan-like motion were repeated continuously, a cur- 

 rent of water sufficient for the purposes of respiration would be kept flowing into and out of the 

 mantle cavity. 



That such a motion of the crura does actually take place is proven by a passage which I 

 shall quote from Willey. 



"There is a slight error in Mosei.ey's account of the movements of the Nautilus, which may 

 as well be corrected. He says, "On cither side of the base of the membranous operculum-like 

 headfold the fold of the mantle closing the gill cavity was to be seen rising and fall- 

 ing, with a regular pulsating motion, as the animal in breathing took in the water, to be expelled 

 by the siphon." It is not a fold of the mantle which is thus seen to pulsate, but the po.sterior free 

 membrane-like expansion of the funnel on either side." 



The current of water caused by the movement of the crura scarcely seems capable of propel- 

 ling the Niiutilus during vigorous swimming. In view of the fact that Willey and Mosely, who 

 have seen the living Nautilus swim, say nothing of the manner in which this movement is effected, 

 it would be pi'esumptuous for one like myself, who has only seen the preserved animals in a labo- 

 ratory far distant from their native haiuits, to form theories as to the swimming movements. I 

 should like to point out, however, that the water in the mantle cavity could be forced out by the 

 withdrawal of the head end of the animal into its shell, caused bj' the contraction of the shell 

 muscles. 



If at the same time the crura of the f uimel be set so as to close the lateral portions of the 

 mantle cavity, the expelled water must pass through the funnel and might propel the animal. 

 The thinness of the mantle and its relation lO the shell suggests, at least, that its contraction is 

 not a factor in the propulsion of the animal. 



MosELEY says that in swimming the various sets of tentacles are extended I'adially from the 

 head. Willey corroborates this account. 



In regard to the Nautilus's power of moving up or down in the water, Willey says: "I have 

 never found any necessity for framing an elaborate theory as to the rising and sinking of the 

 shell. A remarkably small weight is sufficient to sink such an empty shell, and when the living 

 animal retracts itself and ceases all muscular action, thereby converting itself, as it were, into a 

 dead weight, it is heavy enough to sink several shells in addition to its own." 



The air chamber.^ of the shell seem to support the shell itself, relieving the animal of the 

 hindrance which the weight of the shell would otherwise be. That the living Nautilus would 

 float or sink according as it moved or remained rjuiet was suggested as the result of careful com- 

 putations by Meigen thirty 3'ears ago. 



ALIMENTARY SYSTEM. 



It may be well to give a short general description of the alimentary system liefore passing 

 to a detailed description of its various parts. 



The buccal mass forms a large, bluntly conical mass, situated within the various groups of 

 tentacles. (Fig. 7, B.) It is armed with an enormous pair of jaws, which are looked upon still 

 more respectfully when one dissects their strong musculature. The upper jaw tits snugly within 

 the projecting lower jaw, while the tips of both jaws are beaked and are reinforced by a consid- 

 erable deposition of extremel}' hard calcareous material. Upon the floor of the buccal cavity 

 is the large tongue, bearing a long-toothed radula. (Fig. 32, Tn.) Two fleshy folds arise from 

 the floor of the buccal cavity in front of the tongue. (Fig. 3^, AP, PP.) The salivary glands are 

 contained in two larger folds which are situated upon either side of the tongue. (Fig. 32, SP, SO.) 

 The oesophageal opening is immediately behind the tongue. The oesophagus is long and 

 extremely distensible; when full it forms a large pear-shaped crop, with thin, smooth walls; 

 empty and contracted it is a narrow, folded tube. (Figs. 7 and 27, Oe.) The oesophagus passes 

 straight back through the body, finally entering a small chamber which acts as a common vesti- 

 bule to the stomach and the intestine. (Fig. 32, V.) The muscular stomach (Fig. 27, St) lies 



