160 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



indicate that the Nautilus does not trouble itself in this way. but swallows shells and all, leaving 

 the separation of food from refuse to the operations of digestion. Nevertheless, if the radula is 

 excluded from any part in gathering food, it may be of extreme importance in the swallowing of 

 the food. The sides and roof of the mouth are formed by the inner flange of the upper jaw, and 

 consequently the activity of the organs of the floor of the mouth alone must carry the food to 

 the opening of the oesophagus. The radula may seize fleshy food and hold it while it is being 

 bitten: it would also prevent partly bitten food from escaping while the jaws take a new hold. 

 After the food had been bitten off the radula would certainly pull it back upon the surface of 

 the tongue, which, with possibly some aid from the salivary processes, would press the food 

 back into the opening of the (esophagus. 



It is probable that when the radula is in use and consequently under tension the long, 

 lateral teeth are erected so that they no longer cover more central teeth, and are in better 

 position for holding any substance firmly. 



The function of the processes in front of and at the sides of the tongue is also problematical, 

 if we. do not use a still stronger term. It is reasonable to suppose that at least one of their func- 

 tions is to aid in the swallowing of the food. While we do not suppose that the processes at the 

 sides of the tongue have been developed for the sake of bearing the salivary glands, these glands 

 are now much more advantageously situated on account of the high tongue, to aid in the 

 deglutition of food than they would be were they in the floor of the mouth. 



Steinmann's paper on the formation of calcareous matter by the mollusca suggests another 

 possible function of the processes in front of the tongue (especially the anterior one) and the 

 buccal membrane. The decaying secretions of these parts combining with calcium salts of the 

 sea water may form the calcareous matter which covers the tips of both jaws. This is the only 

 explanation I have found which seems to be at all adequate to account for the formation of so 

 much calcareous matter at this point. For there is here no closely applied epidermis, as at the 

 edge of the shell, which could be supposed to take an active part in the formation of this material. 



The (esophagus leaves the buccal mass at its posterior end and so low down as to lie almost 

 central. It immediately passes through the nerve ring as a small, round tube 5 millimeters in 

 diameter. The lengthy (esophagus runs straight back through the hamiocoel to the stomach. 

 (Fig. 27.) The portion of the (esophagus between the nerve ring and the stomach is extremely 

 distensible and forms a crop in which a large amount of food can be stored and gradually passed 

 to the stomach to undergo trituration. When completely tilled the crop forms a large pear- 

 shaped sac with smooth, thin walls. But when it is only partly tilled or is empty the crop shrinks 

 in size and the walls thicken, becoming folded internally with close, longitudinal folds. In the 

 specimen figured (Fig. 32) the posterior part of the crop contained a little food while the remain- 

 ing portion was empty, so both conditions of the wall have been shown. The folds of the anterior 

 portion of the oesophagus are more permanent, probably disappearing more or less only when 

 large pieces of food are being swallowed. The opening in the nerve ring through which the 

 (esophagus passes is so small that it is difficult to imagine how some of the large pieces of food 

 found in the crop could have passed through without exerting a considerable pressure upon the 

 ganglia. At the posterior end of the crop are several longitudinal folds which end abruptly 

 within the opening into the vestibule of the stomach and seem to be more permanent than the 

 other folds mentioned above. (Fig. 32, X.) 



The stomach, into which the (esophagus opens, is an oval, laterally flattened organ 27 milli- 

 meters by 27 millimeters by 15 millimeters in dimensions. At first sight it appears to lie in the 

 genital portion of the ccelom, but in reality it lies in the hsemoccel, closely covered by a back- 

 wardly projecting pocket of the membrane which separates ccelom from hsemoccel. The pos- 

 terior end of the stomach is supported by a thin ligament extending between it and the gonad. 

 (Fig. 27, GL.) The stomach lies in front of, below, and to the left of the gonad. At the center 

 of each flattened side is an irregularly oval, white, tendinous area about 8 millimeters long, from 

 which the muscles radiate which surround the stomach by a thick wall except at the anterior and 

 posterior ends. At the former, where the (esophagus enters and the intestine leaves, a thin-walled 

 chamber is formed, elsewhere spoken of as the vestibule. The muscles pass from one tendon to 



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