314 Introduction to Animal Morphology. 



sidered as the ventral) surface. The inferior free 

 mantle edge may completely surround the head as a 

 collar (Sepia), or may unite with the body integument 

 antero-laterally, so that the mantle becomes only a 

 latero-posterior duplicature of skin dorsally (pos- 

 teriorly). The lateral integument is often expanded 

 into flat lobes or fins, which may be rounded (Sepiola) 

 or angular (Ommastrephes), terminal or sub-terminal. 

 The mantle may secrete no shell (Octopus), or may 

 have imbedded in it a pair of thin conchiolin plates 

 anteriorly (Cirrhoteuthis). In Loligo there is an an- 

 terior shell [gladius] of conchiolin, inclosed in the 

 mantle, consisting of a central rachis, without (Sepiola) 

 or with two thin lateral wings,* varying in shape. 

 This is formed within a " pen-sac," w^hich is a space 

 formed in the embryo by an upgrowth of a ring-like 

 wall of the mantle, the margins of which close to- 

 gether by later growth. In Octopus the fossa is 

 developed, but its margins never close in. In Sepia 

 there is posteriorly, and united to the horny " pen," a 

 lamellated, soft, calcareous shell, forming the well- 

 known cuttlefish bone (used for tooth powder), each 

 of whose lamellee is separated by an interspace filled 

 by animal matter. 



The extinct Belosepia had a similar shell, but the lamellae 

 were farther apart, separated by hollow intervals. In the ex- 

 tinct Belemnites the interior {phragmocone) is composed of 

 lamellae, separated by chambers which communicate with 

 each other by a canal {siphuncle), outside which a part of the 

 original lamellose sepiiform shell remains as a coating 

 {pro-OS traawi), covered with a horny sheath, and like the shell 

 of Sepia, often ending in a beak or rostrum. The belemnitic 



* These -wiiifrs are broader in the female than in the male. 



