152 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



by these one can observe that the two bands just, mentioned always lie exactly at the edge, of the 

 septum. ' 



This portion of the body wall is marked oil' 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- 

 tributed exclusively to the septal region of the body wall and to the siphuncle. The siphuncular 

 artery is a branch of sometimes the left, sometimes the right, septal artery. A variable number 

 of smaller branches of the septal arteries may also pass into the walls of the siphuncle. The 

 branches of the posterior columellar arteries 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 swims in a manner very much 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 shell 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 the water. 



In the tigure (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 above 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 Dibranchiata 

 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 musculature 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 takes 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 place i There are two conceivable 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 turn 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 dorsal 

 and ventral portions of the mantle cavity. 



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

 previous account of these aponeurotic hands published by me docs not give him proper credit for the description of 

 the " septal contour." I read Willey's previous paper carefully before 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 1 have not the least 

 desi re U i 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 complete an account of the anatomy of nautilus as possible, I have purposely refrained 

 in must cases from referring in the 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 this work. 



needless. 



