678 MOLLUSC A— CEPHALOPODA phylum vi 



arranged muscle fibres ; they are also occasionally furnished with horny hooks 

 or sharp claws. Each sucker is able to create a partial vacuum by pressing 

 the cartilaginous rim against some object and then contracting the inner 

 folds, and hence can be used like a cupping-glass. 



The jaws resemble those of Nautiloids in form, but are never calcified ; 

 and owing to their perishable nature, usually horny, they are not preserved in 

 the fossil state. The cartilage of the head forms a complete ring enclosing 

 the central portion of the nervous system. The eyes are of large size, 

 protected by a capsule, and recall those of vertebrates in structure. 



The body is constricted at the mantle opening, which occurs just behind 

 the head, and at this point on the ventral surface is placed the respiratory 

 orifice, bounded by a projecting fold of the mantle. Here also terminates 

 the cylindrical or conical funnel, on either side of which lie the dendriform 

 gills ; in this region also are placed the anal and genital openings. 



The abdomen is sac-shaped, and contains besides the viscera and circu- 

 latory systems a rather large pyriform vessel called the ink-bag. Its reservoir 

 is filled with an extremely opaque brownish-black fluid, which can be voided 

 at will through an excurrent canal terminating near the anus. Menaced or 

 alarmed, the creature discharges a dense cloud of ink, which serves to 

 conceal its retreat. One often finds within the body of fossil Dibranchiates 

 not only a cast or mould representing the ink-bag, but often a dark-coloured 

 residuum of the carbonaceous particles suspended in the ink. 



The abdomen is completely covered by the mantle, which is a thick and 

 frequently brilliantly coloured muscular envelope. Traces of it are occasion- 

 ally found among fossil forms, owing to a slight secretion of calcareous matter 

 within it. 



Most Dibranchiates secrete an internal shell within the mantle. Only 

 among the Octopoda is a shell absent entirely, or replaced in the female by a 

 thin, simple, unchambered spiral ; but this last is in nowise homologous with 

 the usual Dibranchiate shell. Spirula has a spiral, camerated shell, the septa 

 of which are traversed by a siphuncle, and the coils are not in contact. It 

 is situated in the hinder portion of the body and is partially enveloped by the 

 mantle. Among extinct Belemnites the internal shell consists of three parts : 

 a chambered cone {phragmacone), which is prolonged forwards on the dorsal 

 side into a delicate corneo - calcareous proosfracum, and is inserted at the 

 posterior end into a finger-like calcareous piece called the guard (sheath or 

 rostrum) (Fig. 1314, C). 



Some living Cuttle - fishes have a horny, elongated-oval, feather-shaped 

 pi-oostracum or "pen" (Fig. 1332), which is situated dorsally in a closed sac 

 of the mantle. It is sometimes extremely thin, and composed of conchyolin 

 or lime carbonate. The sepion, gladius, or " cuttle-bone," as the shell is called 

 when calcified in some genera, exhibits at its posterior end a small point (the 

 mucro) corresponding to the guard in Belemnites, and extends in front as a 

 broad shelly plate, like a proostracum. This forward extension, when viewed 

 from the front side, is seen to be covered by a mass of thin shelly lamellae, 

 which correspond to the septa more distinctly observed in Belosepia. 



Many living Dibranchiates are gregarious, and swim in the open sea in 

 hordes ; others creep on the bottom or lead a separate existence along rocky 

 shores. They are extraordinarily active, voracious animals, and prey upon 

 mollusks, crustaceans and fishes. A few species are esteemed as food by man. 





