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. 



FIG. 291. 



Embryo of Sepia officinalis, on its vitellus, left-side view, an, anus ; br, gill ; i.lo.l, invagina- 

 tion of the lateral cerebral lobe ; 710, fin ; pa, mantle ; vit, vitellus. I, II, III, IV, V, the five 

 left arms. 



IV. BIONOMICS 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 Xaiitilits, 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 



