280 MOLLUSCA. CEPHALOPODA 



consequently seldom preserved in fossil specimens. An 

 ink-sac is always present, and is sometimes found fossil. 

 The funnel is in the form of a complete tube. 



A shell is absent in some forms ; when present it is 

 (except in Argonauta) internal, and may be either horny 

 or calcareous. In some cases (Sepia) it has the form of 

 an oval flattened body, known as the cuttle-bone, which is 

 composed mainly of laminated calcareous material with 

 spaces between the laminae. In the squids the shell is 

 lamellar in form and consists of horny material ; it is 

 termed the pen or gladius. The shell in the cuttle-fishes 

 and squids is placed on the antero-dorsal side of the body 

 in a sac formed by the mantle. In Spirula the shell 

 resembles that of a Tetrabranch, but is internal, being 

 almost entirely covered by the mantle ; it is situated at 

 the dorsal end of the body, and consists of a tube coiled in 

 a plane spiral and divided into chambers by septa, which 

 are traversed by a siphuncle placed near the inner margin; 

 the whorls are not in contact, and a calcareous protoconch 

 is present. The shell in the paper-nautilus (Argonauta) 

 is of quite a different nature to that found in other 

 Dibranchs ; it is external and spiral, but not chambered, 

 and is without muscular attachments ; it is secreted by 

 the terminal portions of the two anterior arms, and is 

 found only in the female, serving for the reception of the 

 eggs. 



The Dibranchia are divided into two sub-orders : — 

 (1) the Decapoda, (2) the Octopoda. 



