40 THE LIFE OF THE MOLLUSCA 



Octopods there is no longer any true shell, but only 

 some simple chitinous rudiments, to which the 

 retractor muscles of the head and funnel are 

 attached. In certain fossil Dibranchia (Belemnites, 

 etc., Plate XXIII.) the internal chambered shell, 

 known as the " phragmocone," is enclosed in a 

 pointed calcareous sheath, or " guard," at the end 

 opposite to the head, while from the dorsal margin 

 of the anterior end of the phragmacone there arises 

 a broad, thin, chitinous plate, called the " pro- 

 ostracum." To modifications in form of these three 

 constituent parts, or to the partial or total suppres- 

 sion of one or more of them, the resultant differences 

 between the internal shells of the other members of 

 the order may be traced. This, however, will be 

 better gathered from the diagrammatic figures on 

 Plate XXIV. than from any lengthy description. 



The shell of the Paper Nautilus, or Argonaut 

 (Plate XXL, Fig. 4; Plate XXV., Fig. 12), stands 

 on a different footing to the ordinary shell, and does 

 not originate in the shell-gland; it is only formed 

 some days after the creature is hatched, and is 

 peculiar to the female, being chiefly used as a 

 vehicle for carrying and protecting the eggs, which, 

 when deposited, are agglutinated to it. It is exo- 

 gastric, and composed of three layers, of which the 

 outer and inner are alike and prismatic, while the 

 middle one is fibrous : there is no pearly layer 

 within. The animal is not attached to it in any 



