824 



CEPHALOPODA. 



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, 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 



Fig. 737- — a, Internal skeleton of Sepia omata ; b, Pen of Histiotenthis Boiielliana , 

 c, Shell (" phragmacone ") of Spirulafragilis ; d, Animal of Spirula Peronii. 



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 Tetrabranchiata, 

 in accordance with the number of gills possessed by the animal. 

 The former comprises all the true Cuttle-fishes ; whilst the latter, 

 though abundantly represented in past time, has no other living 

 representatives than three or four species of the genus Nautilus. 



As regards their distribution in space, the Cephalopods are ex- 

 clusively inhabitants of the sea, and sometimes attain a great size. 

 They live partly in the open sea and partly in shallow water close 

 to shore, and are carnivorous in habit. 



As regards their general distribution in time, the oldest forms of 



