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The animal of the Nautilus has a large mantle or fleshy sac 

 enclosing the internal organs, which can be opened around the 

 margin, or closed, at the will of the animal. Admitting the water 

 around the margin they fill their mantle cavity with fluid, and then 

 constricting the margin and compressing the mantle-sac, force it 

 out with violence through a fleshy pipe, which is exclusively used 

 for that purpose, and always situated on the ventral side. The reac- 

 tion of the stream is sufficiently powerful to drive the body of the 

 animal with varying degrees of swiftness backwards. The fleshy 

 pipe is therefore an ambulatory pipe or hyponome ; and it is advan- 

 tageous to replace the old and confusing terms by this name. 



The Dibranchiata change the external shell, which they inherit 

 from the Nautiloids, into an internal organ, and by suitable modifi- 

 cations of shape and also taking advantage of the powerful hydrau- 

 lic apparatus, which they also inherit, and increasing its efficiency, 

 become exclusively swimmers. 



The hyponome of the Nautilus causes a corresponding depres- 

 sion or sinus to occur in the aperture of the shell on the same side, 

 and its effect is also to be seen in the striae of growth on this side ; so 

 that we know, from these indications in any fossil, what was the 

 comparative size of the pipe, and whether the animal was more or 

 less powerful as a swimmer. 



Other indications, such as the openness or contracted form of 

 the various apertures of different genera, exhibit with equal clear- 

 ness what they could do in the way of crawling. The wide-open 

 apertures indicate powerful arms, capable of carrying and easily 

 balancing the large spire of the shell above ; the narrow contracted 

 aperture shows that the arms were small, and that the animal 

 could not so efficiently balance or carry the shell in an upright 

 position, and was therefore, according to the amount and style of 

 the contraction, more or less inefficient as a crawler. 



In studying the different types of the Tetrabranchiata, we find 

 that there are two orders as first defined by Prof. Louis Agassiz — 

 the Nautiloidea and the Ammonoidea — and, further, that these divi- 

 sions coincide with differences in the outlines of the ambulatory 

 sinuses which indicate distinctions of habit general in the normal 

 forms of each order. 



The extinct Nautiloidea had large ambulatory sinuses, and were 

 evidently capable, like the modern Nautilus, of rising to the sur- 



