THE PAST AND PRESENT OF THE CUTTLEFISHES. 105 



the vast majority, some 300 species, or so, being included in the ranks 

 of living animals. The cuttlefishes are very widely distributed in 

 existing seas. They occur in the far north ; they are plentifully 

 represented in the colder seas by the squids which form the bait of 

 the Newfoundland cod-fishers; but in tropical regions they attain 

 their greatest size and numerical strength. Their classification is 

 both simple and natural. Their division into Dibranchiates (" two- 

 gilled") and Tetrabranchiates ("four-gilled") is a method of arrange- 

 ment which accurately reflects variations in their existing structure, 

 as it correctly indicates the main lines of their geological and past 

 history. Of four-gilled cuttlefishes there is but one living example 

 the pearly nautilus (fig. 8). Its special and distinctive peculiarities 

 may be rapidly summed up in the statement that it has four gills, 

 numerous arms (c\ no suckers, no ink-sac, an incompletely tubular 

 funnel (/), stalked eyes, and an external many-chambered shell, in 



FIG. 8. PEARLY NAUTILUS. (Shell in section.) 



the last-formed and largest compartment (<?) of which the body is 

 lodged. 



The absence of an ink-sac in the nautilus is a fact correlated 

 with its bottom-living habits and with the absence of any need or 

 requirement 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-chambered state is explained by the fact that as the animal 

 grows it successively 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 (g). Through the middle of the series of 

 septa runs a tube named the sipuncle (s, s), whose function has been 



