CLASSIFICATION OF CEPHALOPODA. 385 



Classification of Cephalopoda. 



Order I. Tetrabranchiata (see Table). 



Family I. Nautilidse. Nautilus alone alive ; but a great 



series of fossil forms, Orthoceras — Trochoceras. 

 Family II. Ammonitidse. All extinct, but with shells well 

 preserved, so that long series can be studied. They 

 furnish stiiking evidence of progressive evolution in 

 definite directions, e.g. Bactrites, Ceratites, Baculites, 

 Turrilites, Heteroceras, and the whole series of genera 

 formerly classed as Ammonites. 

 Order II. Dibranchiata (see Table). 



Sub-Order Decapoda. Eight shorter and two longer arms. 

 Suckers stalked and strengthened by a strong ring. 

 Large eyes with a horizontal lid. Body elongated, with 

 lateral fins. Mantle margin with a cartilaginous " hook- 

 and-eye " arrangement. Some sort of internal " shell," 

 enclosed by upgrowths of the mantle. 

 With calcareous internal "shell." Spirula; extinct Bel- 



emnites ; Sepia. 

 With organic internal ' ' shell. " 



(a) Eyes with closed cornea, e.g. Loligo. 

 (d) Eyes with open cornea, e.g. Ommastreplies. 

 Sub-Order Octopoda. Eight arms only. Suckers sessile 

 without horny ring. Small eyes with sphincter-like 

 lid. Body short and rounded. No "hook-and-eye" 

 arrangement. No "shell," except in the female 

 Argonauta. 



e.g. Octopus, Eledone, Argonauta. 

 The classification given above is that usually adopted, but it is not 

 certain that the Ammonites should be included in the Tetrabvanchiata. 



The Cephalopods are the most specialised of the Molluscs, 

 and present much variation of type. Nautilus appeared 

 very early and has persisted, apparently unchanged, until the 

 present, while the Ammonites and Belemnites, once so 

 abundant, have entirely disappeared. Among recent forms 

 we have Squid, Calamary, Octopus, Argonaut, and many 

 others. All swim freely in the sea, or lurk and creep 

 passively among the rocks. They are voracious eaters, and 

 devour very diverse kinds of animals, their parrot-like jaws 

 and powerful odontophore, as well as the numerous suckers, 

 rendering them formidable adversaries. Many live at 

 considerable depths, and their chief foes are the toothed 

 whales, some of which, like the sperm whale {Physeter), 

 and the bottle-nose {Hyperoodon), subsist almost entirely on 

 cuttles. 

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