152 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



by these one can ob.sorve that tlie two l)aiids just luciitioned always lie exactly at the edije of the 

 septum." 



This portion of tiie l)ody wall is inai-ki>d off from the rest by its arterial vessels also. (Text- 

 tig. 11, p. 186.) The septal artery runs backward from the heart, and entering the body wall 

 below and a little to the left of the base of the siphuncle divides into two branches which are dis- 

 trilnited (>xclusi\ely to the septal region of the body wall and to the siphuncle. Th(> sipluuicular 

 artery is a branch of sometim(>s the left, sometimes the right, septal artery. A \ariablc number 

 of smaller l)ranches of the septal arteries may also pass into the walls of the siphuiu-le. The 

 branches of the posterior columellar arteri(>s to the dorsal body wall seem also to enter the 

 septal area, but I can not be sure of their distribution without injected specimens. Willey 

 states that '"the septum -producing area of the mantle" is distinguished in fresh specimens 

 "from the surrounding portions of the mantle by its greater thickness and opacity." Such a 

 distinction can not be observed in alcoholic specimens. 



MOVEMENTS OF SWIMMING AND RESPIRATION. 



Nautilus swuus in a manner vtny nuich like that of the Dibranchiata, backward (or with the 

 posterior end of the body pointed in the direction of motion), propelling itself by means of jets 

 of water squirted through the funnel. On account of the presence and the position of the air 

 chambers of the sliell the involution of the latter is always dorsal. Willey has already clearly 

 stated that on this account it would be impossible for the animal to turn over in tb(> water. 



In the figure (a photograph) which Willey gives of a living Nautilus in the swimming 

 position the anterior end of the body is raised .so that the eyes are al)ove the edges of the shell, 

 and the back of the hood nearly covers the dark portion of the involution. of the shell, apparently 

 projecting higher upon this than the dorsal portion of the mantle. 



When we are told that the manner of progression of Nautilus is like that of the Di))ranchiata 

 we involuntarily imagine that the water is expelled from the mantle cavity by the contraction of 

 the mantle. 



Possibly this is the case, but the scantiness of the nuisculature of the mantle seems to afford 

 good ground for doubt. It has been pointed out in some cases that a chitinization of portions of 

 the mantle tak(>s place, which would surely interfere with its contraction. 



Apparently also the mantle is attached to the edge of the shell. If this is true it is impossible 

 that the mantle by its contraction should drive the water from the mantle cavity. 



How, then, can the expulsion of water take placed There are two conceivat)le ways, one of 

 which has been observed. 



We have noticed that the crura of the funnel extend upward along the sides of the body 

 from the posterior end of the funnel. They are attached to the body by one edge; in expanded 

 specimens the crura are seen to gradually thin out from the attached edge to a very thin, pli- 

 able free edge. 



If the crura should be set so that their surfaces form a large angle with the body, the outer 

 free edges would be in contact with the inner surface of the mantle, and thus the mantle cavity 

 would be completely closed except for the passage through the funnel. The dorsal ends of the 

 crura tuiMi along the upper sides of the shell muscles toward the posterior limit of the, here very 

 narrow, mantle cavity. These ends would serve to close the communication between the dor.sal 

 and ventral portions of the mantle cavity. 



' In Part VIII, Vol. IX, of the Proceedings of the CanibriJge Philosophical Society, p. 398, Willey claims that a 

 previous account of these aponeurotii- hands published by me does not give him proper credit for the description of 

 the "septal contour." I read Willey's previous paper carefully liefore writing my first description, _and have read it 

 several times since. He does not in any way describe the manner of limitation of the septal area. It seems, therefore, 

 that my description completes his without in any way intrenching upon his priority, a thing I have not the least 

 desire to do. It may be well to state that in gathering the accounts of many authors into one and incorporating them 

 with my own work to form as c(jmplete an account of the anatomj' of nautilus as possible, I have purposely refrained 

 in most ca.ses from referring in tlie text to the original descriptions of the various parts. Those who are familiar with 

 Cephalopod literature will give credit where credit is due. References in the text are, for the purposes of tlus work. 

 needless. 



