432 PHYLUM MOLLUSCA. 
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.” Sfzvula; extinct Bel- 
emnites ; Sepia, 
With organic internal ‘ shell.” 
(a) Eyes with closed cornea, Myopsida, e.g. Loligo. 
(4) Eyes with open cornea, Oigopsida, ¢.g. Ommeastrephes. 
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, 
eg. Octopus (Polypus), Eledone (Moschites), Argo- 
naula, Ctrroteuthts (with cirri on the arms and 
no radula). 
The classification given above is that usually adopted, but it is not 
certain that the Ammonites should be included in the Tetrabranchiata. 
The Nautiloids began in the Cambrian and died out at the end of 
the Paleozoic period, except the Orthoceras and MNautclus-like types. 
The genus Vauti/us appeared in the Cretaceous. The Ammonite series 
lasted from the Silurian to the early Tertiary. 
The Cephalopods are the most specialised of the 
Molluscs, and present much diversity of type. They swim 
freely in the sea, or creep sluggishly 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 formid- 
able adversaries. Many live at considerable depths, and 
their chief foes are the toothed whales, some of which, like 
the sperm whale (Pfyseter), and the bottlenose (Ayperoo- 
don), subsist almost entirely on cuttles. Some deep-sea 
forms have highly evolved luminous organs. 
Shells of Cephalopods.—A chambered external shell, serving as 
a house, is present in Mazedéz/us alone among living Cephalopods. 
Most of the extinct forms had large and efficient shells of very 
diverse shape, some straight like Orthoceras, or coiled, with chambers 
separated by complex septa, as in the Ammonites. In the majority 
of shells of the Ammonitid series, the septa between the chambers 
are convex towards the aperture (the opposite in the Nautilus); the 
siphuncle is marginal or ventral; the septal necks of the siphuncle 
project forwards (not backwards as in the Nautilus); there is 
