SUB-CLASS II 



Dir.KANclllATA 



The body of Dibranchiates or Cuttle-fishes is elongated, cylindrical, or 

 sack-shaped, and frequently provided with two lateral fin-like appendages. 

 The anterior cephalic region gives off a circlet of eight or ten powerful, 

 muscular arms, the inner sides of which are armed with suckers (an-f"l> 

 a double row of hooks, and assist in swimming or creeping, and also serve for 

 the capture of prey. The Sepioulea have two of 

 their ten arms developed into very long tentacles 

 which bear hooks or suckers only at their thickened 

 extremities (Fig. 1236). The lower surface of 

 the suckers is disk- or cup-shaped, perforated in 

 the middle, and occupied by numerous radially 

 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 ; 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 a 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 respira- 

 tory 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 the same neighbourhood too S*l8U&Btt. At V 

 are placed the anal and genital openings. 



The abdomen is sack-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 an impenetrable 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 Odopoda 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. Spirilla has a spiral, camerated shell, the septa 

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

 VOL. I 2 Q 



FIG. 1236. 

 Enoploteuthis leptura. Recent; 



