MOLLUSCA: CEPHALOPODA. 



425 



segmentation, as in Birds and Reptiles, and the unsegrnented 

 portion of the yolk is gradually absorbed by the growing 

 embryo. 



The shell of the Cephalopoda is sometimes external, some- 

 times internal. The internal skeleton (fig. 229) is known as 



Fig. 229. a Internal Skeleton of Sepia ornata", b Pen of Histioteuthis Bonelliana ; 

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



the " cuttle-bone," " sepiostaire," or " pen " (gfadius), and may 

 be either corneous or calcareous. In some cases it is rendered 

 complex by the addition of a chambered portion or " phrag- 

 macone," which is to be regarded as a visceral skeleton or 

 " splanchnoskeleton." In Spirula (fig. 229, c) the phragma- 

 cone 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 internal, and differs, therefore, in this respect 

 from the external shell of the latter, though resembling it in the 

 fact that the last chamber lodges part of the viscera. 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 structure of 

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



