2/4 MOLLUSCA. 



shell, and this means of defence is not needed, there is no ink- 

 bag. 



The shell of the Cephalopoda is sometimes external, some- 

 times internal. The internal skeleton is known as the " cuttle- 



Lower Greensand (Cretaceous). 



bone," " sepiostaire," or " pen " (gladhis), and may be either 

 corneous or calcareous. In some cases it is rendered complex 

 by the addition of a chambered portion or " phragmacone," 

 which is to be regarded as a visceral skeleton or " splanchno- 

 skeleton." In Spirula the phragmacone is the sole internal 

 skeleton, and is coiled into a spiral, the coils of which lie in 

 one plane, and are near one another, but not in contact. It 

 thus resembles the shell of the Pearly Nautilus, but it is inter- 

 nal, and differs, therefore, entirely from the external shell of the 

 latter. The only living Cephalopods which are provided with 

 an external shell are the Paper Nautilus (Argonauta), and the 

 Pearly Nautilus (Nautilus pompilius) ; but not only is the struc- 

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

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

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

 (fig. 248), on the other hand, is a true pallial shell, and is se- 

 creted 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 



