NAUTILUS. 



Plate II. 



Fig. 1 b. (Mus. nostr.) 



Nautilus Pompilius. A section showing the cham- 

 bered interior and siphonic tube. The Nautilus shell, it 

 may be observed, is composed of three contiguous, ra- 

 pidly enlarging whorls, convoluted in a discoid manner 

 round a nucleus, which leaves an umbilical orifice per- 

 forating the centre. In N. Pompilius the umbdicus is 

 concealed from view when the shell arrives at maturity, 

 by an overlaying of callous pearly matter ; but in the 

 other species it is exposed in the adult. When the shell 

 is held before the light, the observer may clearly see 

 through the umbilicus. The interior of the shell is par- 

 titioned into from thirty to five-and-thirty chambers, and 

 the partitions or septa are concave, perforated in the mid- 

 dle by a narrow siphonic tube. What, then, are the uses of 

 these chambers, and of this siphonic tube? The tube en- 

 closes a ligament, the dried remains of which may often 

 be seen in sections, especially in the small central cham- 

 bers, passing through the entire spiral, and attached to the 

 body of the animal. Some writers are of opinion that 

 the septa being formed periodically, the animal slips its 

 muscular attachment at such intervals, and lowers itself, 

 as it were, to the distance required for the formation of 

 another chamber by a dilation of the ligament. The im- 

 pression of the muscular girdle may be seen in the ac- 

 companying figure at the bottom of the outer porch of the 

 shell, and traces of the attachment from whence it has 



periodically slipped are visible in the previously formed 

 chambers. Other writers incline to the belief that the 

 muscle of attachment grows only in front, and wastes be- 

 hind, and is always moving forward during the period of 

 growth, except at the intervals of the formation of the 

 septum. 



The use of the chambers is also a matter of specula- 

 tion. One thing is certain; they can have no hydro- 

 static function for the purpose of enabling the animal to 

 dive into the depths of the sea or rise to its surface. 

 They are most probably air-chambers destined to assist 

 the specific gravity of the animal. The natural position 

 of the Nautilus, when crawling on the bed of the ocean, 

 at a depth, probably, of from twenty to thirty fathoms, is, 

 as in other Cuttles, with the head downwards, the shell is 

 accordingly uppermost, and the chambers containing ni- 

 trogen gas serve, doubtless, to buoy it up. If the animal 

 enlarges the growth of its shell while in this position, pe- 

 riodically slipping its muscular belt, the shell would natu- 

 rally have a tendency to coil in the form of a disk ; ami 

 the air-chambers would increase in size to meet the in- 

 creasing pressure of the surrounding element, arising from 

 the animal's increase of bulk. It should be noticed that 

 the siphonic tubes of the last four chambers are not seen 

 in the specimen figured, owing to a defect in the cutting. 

 Being cut a little obliquely, they have been carried a nay 

 in the opposite half. 



June, 1861. 



