LECTURES ON MOLLUSC A. 17 



ORDER II. TETRABRANCHIATA, 



or four-gilled cephalopoda, of Professor Owen. It might be thought 

 a matter of little importance whether a cephalopod had one or two 

 pairs of gills ; but it happens that this difference is coordinate with 

 others that run through the whole form and structure of the animals. 

 The two-gilled cuttles, we have seen, are adapted for an active and 

 predacious life. As they could not dart after their prey carrying a 

 heavy shell, they are naked, but furnished with powerful arms and 

 ink-bag for their protection. The four-gilled tribes, on the other hand, 

 are destined for a quieter life, crawling on the ground like common 

 Gasteropods. Instead of eight or ten arms with suckers and hooks, 

 they have a multitude of small retractile feelers, something like the 

 Sea Anemone. On these they can creep, and draw their prey to their 

 mouths ; but they are not able to pursue it in the open sea. Instead 

 of a strong breathing tube with a valve, answering the purpose of a 

 forcing pump and propeller, they have only an open gutter made by a 

 fold in the mantle, like the siphons of the Gasteropods. The eyes, 

 which in the cuttles have optic ganglia much larger than the central 

 brain, (Alcock,) are here less conspicuous, and mounted on peduncles. 

 The head and tentacles, instead of being the principal part of the 

 creature, to which the body might appear subordinate,, are here scarcely 

 separated from it, and retractile within the general mass. They are 

 always furnished with a chambered shell, the last cavity of which 

 contains the animal. When disturbed, instead of squirting ink and 

 darting off, it shrivels up into its cavity and takes its chance. If it 

 sees a delicate crab at a distance, instead of pouncing on it, it must 

 crawl, not, indeed, on "all fours," but on "all dozens;" or wait until 

 the creature comes within seizing distance, when it will be entangled 

 in the arms and be broken up by the jaws or gizzard. 



Only one animal formed after this type is now known to be living 

 on the earth; the pearly or true Nautilus, whose many-chambered 

 shell has been an object of admiring speculation from early times. 

 This is the last straggler belonging to a race which performed import- 

 ant functions in the early ages of our globe. The Nautili themselves 

 are among the few genera which have existed at every period of the 

 world's history. Our knowledge begins with one species from the 

 upper silurian rocks of Bohemia. It has not culminated at any par- 

 ticular period ; not more than seven species appearing in any forma- 

 tion ; but it has never been without its representatives, and two or 

 three species are now crawling on the sea bottoms in the East Indian 

 archipelago. Before them, however, lived the great Orthoceratites of 

 the palaeozoic seas ; and as they died out, the great family of the Am- 

 monites developed themselves, and held possession of the seas till the 

 close of the cretaceous period, when they suddenly disappeared, leaving 

 not even a distant relation to grace the tertiary formations. Coordi- 

 nate with the prevalence of four-gilled Ceplialopods, we find a general 

 absence of the predacious Gasteropods which are now so numerous and 

 highly developed. We may suppose, therefore, that they played the 

 same part in the economy of nature ; and that the Orthoceratites and 



