CLASS VI. CEPHALOPODA, Cuvier. 



THE Cephalopoda are perfectly symmetrical Mollusca in which the 

 edges of the foot are transformed into circumoral appendages com- 

 pletely surrounding the head, and the epipodium is modified to form 

 an exhalant muscular tube or funnel consisting of 2 free or united 

 lobes, situated behind the head, at the opening of the pallial cavity. 

 In the nevous system all the typical ganglion-pairs are concentrated in 

 the head, and are applied to or contained in the interior of a carti- 

 laginous skeletal piece. The renal organs are constituted by the 

 glandular covering of the afferent branchial vessels. The ccelom 

 communicates with the exterior either directly or by the intermediary 

 of the paired kidneys, and by a second pair of ducts serving as gona- 

 ducts. The gonad is situated in the ccelom, and is not continuous 

 with the gonaducts. A portion of the circumoral pedal crown is 

 ' hectocotylized ' : that is to say, modified to form a copulatory 

 organ in the male. The development is characterized by the incom- 

 plete segmentation of the ovum. 



In comparing the Cephalopoda with other Mollusca, one finds that 

 the ventral surface is much abbreviated and the length of the body 

 reduced. This is the result of the displacement of the foot, whose 

 lateral borders surround the head and are joined together in front of 

 the mouth. In consequence of this shortening of the antero-posterior 

 axis, the two extremities of the digestive canal are closely approxi- 

 mated, and the pallial cavity opens immediately behind the head. 



The head is highly developed, but has hardly any other appendages 

 than those formed by the edges of the foot which embrace it. The 

 foot forms a crown of appendages surrounding the mouth, which 

 in the Dibranchia have the form of 4 or 5 pairs of symmetrical and 

 generally elongate arms. In the Octopoda there are 8 similar arms, 

 and the whole length of the ventral surface of each is covered by 

 suckers, which are often very numerous and highly specialized in 

 structure. In the Decapoda, in addition to the 8 arms corresponding 

 to those of the Octopoda, there are 2 additional " tentacular " arms, of 

 which one is situated between the third and fourth sessile arms on 

 either side of the posterior part of the head. These 2 tentacular 

 arms are longer and more slender than the others, and the suckers are 

 generally confined to their free extremities, which are enlarged and 

 club-shaped ; in some forms, however, they bear suckers along their 

 whole length. The tentacular arms are further distinguished from 

 the sessile arms by the fact that they are more or less retractile within 

 special pouches. In the female Argonaula the 2 dorsal arms are en- 

 larged to form a veil, which is applied to the mantle and secretes 

 a protective calcareous shell. 



In addition to the foot proper, whose edges constitute the circum- 

 oral appendages, the Cephalopoda possess an epipodium which is 



