MOLLUSCA : CEPHALOPODA. 



303 



sequently it was described as the entire male, and it is only 

 recently that its true nature has been fully ascertained. 



Fig. 108. i. Octopus carena (male), showing cyst in place of the third arm. 2. Ven- 

 tral side of an individual, more developed, with the hectocotylus (a). (After 

 Woodward.) 



The shell of the Cephalopoda is sometimes external, some- 

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

 bone," " sepiostaire," or "pen" (gladius), 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 internal, 

 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 (Argonautd), 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 



