442 TEXT-BOOK OF ZOOLOGY 



CLASS III. : CUTTLE-FISHES (CEPHALOPODA). 



Perfectly bilaterally symmetrical molluscs -with distinct head ; with 

 arms which surround the mouth, and are provided with suckers ; with 

 muscular lobes, the lateral margins of which are applied to each other 

 behind, and mostly fuse so as to form a funnel. 



ORDER I. : DIBRANCHIATA. 



The Common Cuttle-Fish {Sepia officinalis). 



(Length without tentacles up to 12 inches.) 



A. Topography of Structure. 



The cuttle-fish possesses a large head definitely distinguished from the 

 body, and furnished with a number of so-called " arms " (Cephalo- 

 poda = head-footed) which surround the mouth, and on their inner 

 surface are covered with suctorial discs. The head bears on each side 

 a pair of very large eyes. The body is flattened, and carries on each 

 side an integumentary fin. On the dorsal side the skin encloses a plate- 

 like shell of loose texture — the so-called cuttle-bone. On the ventral 

 surface the mantle forms the mantle cavity, or pallial chamber, from 

 which protrudes the perforated foot, or so-called funnel. Thus, in its 

 structure the cuttle-fish presents essential differences from the gastropods 

 and lamellibranchs. Nevertheless, as in the case of the last-named 

 groups, we discover the same intimate connection between its — 



B. Structure, Habitat, and Mode of Life. 



1. The cuttle-fish is an inhabitant of the sea. It is found abundantly 

 everywhere on the coasts of the Mediterranean and Western Europe, and 

 in many places forms an important object of the fishing industry. (The 

 flesh is eaten ; the contents of the ink-bag — see Section 5 — furnish an 

 important painter's colour, viz., sepia ; and the cuttle-bone is used as a 

 polishing material.) The animal is also occasionally found in the North 

 Sea. 



2. Respiration. — In accordance with its habitat, the cuttle-fish 

 breathes by gills, which form two delicate plume-like structures, lodged 

 in the branchial or pallial cavity. The water enters this cavity at the 

 free edge of the mantle, bathes the gills, and is again expelled, together 

 with the faces and the excretions of the kidneys, which, as well as the 

 intestine, open in the mantle cavity. These various materials are 

 expelled by the tubular funnel, which protrudes from the mantie cavity 



