mollusca: CEPHALoroDA. 327 



ture of the animal different in each of these, but the nature of 

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

 (fig. 117) 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 hve 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. 119), 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. 



The Cephalopoda are divided into two extremely distinct and 

 well-marked orders, termed the Dibranchiata and the Tetra- 

 branchiata. The former is characterised by the possession of 

 two branchiK only, and comprises the Cuttle-fishes, Squids, 

 and the Paper Nautilus. The latter is distinguished by the 

 presence of four gills, and, though abundantly represented in 

 past time, has no other living representative than the Pearly 

 Nautilus alone. 



CHAPTER LI. 



DIVISIONS OF THE CEPHALOPODA. 



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 

 branchiae, 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 

 funnel. The arms constitute powerful offensive weapons, 



