Page:Popular Science Monthly Volume 21.djvu/779

Rh running in parallel, but nevertheless in distinct and independent, lines; and this likeness is further strengthened when we discover that not merely the ear, but the eye likewise, of these two groups of animals is formed or developed in an essentially similar fashion. The ear of the cuttle-fish presents us with a permanent example of an early and transitory stage in the development of the vertebrate ear, and a common plan of ear-production is thus seen to traverse a wide extent of the animal world.

The present history of the cuttle-fishes may be concluded by the briefest possible reference to their distribution and classification. Over two thousand species of cephalopods are known. But geology claims the vast majority, only two hundred and eighteen species being included in the ranks of living animals. The cuttle-fishes 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 arrangement which accurately reflects variations in their existing structure, as it correctly indicates the main lines of their geological and past history. Of four-gilled cuttle-fishes there is but one living example—the pearly nautilus (Fig. 3). 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 (f), stalked eyes, and an external, many-chambered shell, in the last formed and largest compartment (e) 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