330 THE CEPHALOPODA 
The vitelline mass diminishes insensibly during the growth of 
the embryo, and is for the most part absorbed at the time of 
hatching. The yolk sac is independent of the stomach, and is only 
in contact with it over a small area in the middle line. 
save i202 
Fic. 291. 
Embryo of Sepia officinalis, on its vitellus, left-side view. an, anus; br, gill; 7.Jo./, invagina- 
tion of the lateral cerebral lobe; na, fin; pa, mantle; vit, vitellus. I, II, III, IV, V, the five 
left arms. 
IV. BIoNomMIcS AND DISTRIBUTION. 
All the Cephalopoda are marine, and very active animals. They 
swim rapidly by expelling the water from the pallial cavity through 
the funnel, sometimes with so much violence that they can spring 
for some distance out of the water (Ommatostrephes). The fins 
of the Dibranchia are organs of balance rather than of locomotion. 
All the Cephalopods are in the highest degree carnivorous: many 
Fig. 292. 
Diagrams of sections showing the early stage of development of the eye of Loligo, when it 
is, like the permanent eye of Nautilus, an open sac. A, first appearance of the eye as a ring- 
like upgrowth; B, ingrowth of the ring-like wall so as to form a sac, the primitive optic 
vesicle. (After Lankester.) 
of them destroy a large number of edible fish and Crustacea, but, on 
the other hand, many of the pelagic forms fall a prey to the toothed 
whales. 
Some Cephalopods attain to a very considerable size: the body 
of some species of Architeuthis, without the head, may measure two 
and a half metres in length, and when the head and extended 
5 rns ia Naa 
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