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THE POPULAR SCIENCE MONTHLY. 



ment for the sudden concealment from enemies which the more active 

 two-gilled forms demand. The many-chambered shell of the pearly 

 nautilus exhibits a flat, symmetrical, spiral shape. Its many-cham- 

 bered state is explained by the fact that as the animal grows it suc- 

 cessively leaves the already formed chambers, and secretes, a new 

 chamber to accommodate the increasing size of body. Each new 

 chamber is partitioned off from that last occupied by a shelly wall 

 called a septum (cj). Through the middle of the series of septa runs 

 a tube named the siphuncle, (s, s), whose function has been credited 

 with being that of maintaining a low vitality in the disused chambers 

 of the shell. 



All other living cuttle-fishes possess, on the contrary, two gills, 

 never more than ten arms jDrovided with suckers, an ink-sac, unstalked 

 eyes, a completely tubular funnel, and an internal shell. If, however, 

 the nautilus represents in its solitary self the four-gilled cuttle-fishes 

 of to-day, it likewise, like " the last of the Mohicans," appears as the 

 descendant of a long line of famous ancestors. In its distribution, 

 the nautilus is limited to the southern seas. It is still the rarest of 

 animals in our museums, although its shells are common enough. 



H \\ 







Fig. 4. Paper Nattilus. A, female argonaut showing shell, around which the two expanded 

 arms are clasped ; B, female removed from shell ; C, the male argonaut (shell-less). 



This, according to Mr. Moseley, is no doubt due to the fact that the 

 animal is mostly an inhabitant of deep water. The shells of /Spirida 

 (Fig. 6) similarly occur in countless numbers on tropical beaches, yet 

 the animal has only been procured two or three times. 



