426 MANUAL OF ZOOLOGY. 



shell itself is entirely different. The shell of the Argonaut (fig. 

 230) is involuted, but is not divided into chambers, and it 'is 

 secreted by the webbed extremities of two of the dorsal arms 

 of the female. The arms are bent backwards, so as to allow 

 the animal to live in the shell, but there is in reality no organic 

 connection between the shell and the body of the animal. In 

 fact, the shell of the Argonaut, being confined to the female, 

 and serving by its empty apex as a receptacle for the ova, may 

 be looked upon as a " nidamental shell," or, as it is secreted 

 by a modified portion of the foot, it may more properly be 

 regarded as a " pedal shell." The shell of the Pearly Nautilus 

 (fig. 233), on the other hand, is a true pallial shell, and. is 

 secreted by the body of the animal, to which it is organically 

 connected. It is involuted, but it differs from the shell of the 

 Argonaut in being divided into a series of chambers by shelly 

 partitions 'or septa, which are pierced by a tube or "siphuncle," 

 the animal itself living in the last chamber only of the shell. 



CHAPTER XLIX. 

 DIVISIONS OF THE CEPHALOPODA. 



THE Cephalopoda are divided into two extremely distinct and 

 well marked orders, termed the Dibranchiata and Tetra- 

 branchiata. The former comprises all the true Cuttle-fishes ; 

 whilst the latter, though abundantly represented in past time, 

 has no other living representative than the Pearly Nautilus 

 alone. 



ORDER I. DIBRANCHIATA. The members of this order of the 

 Cephalopoda are characterised as being swimming animals, al- 

 most invariably naked, with never more than eight or ten arms, 

 which are always provided with suckers. There are two branchia, 

 which are furnished with branchial hearts ; an ink-sac is always 

 present ; the funnel is a complete tube, and the shell is internal, 

 or, if external, is not chambered. 



The Cuttle-fishes are rapacious and active animals, swim- 

 ming freely by means of the jet of water expelled from the fun- 

 nel. The arms constitute powerful offensive weapons, being 

 excessively tenacious in their hold, and being sometimes pro- 

 vided with a sharp claw in the centre of each sucker. They 

 are mostly nocturnal or crepuscular animals, and they some- 

 times attain to a great size. They may be divided into two 



