CEPHALOPODA. 



459 



the Dibranchiata. The shell is often variously ornamented externally with 

 striae, knobs, &c.; it is composed of an outer layer with round or oval 

 calcareous bodies irregularly arranged, and coloured in Nautilus and 

 many Nautiloidea, and an inner layer of nacre or mother of pearl which 

 is laminated and forms the whole substance of the septa. 



The ectoderm consists of a single layer of cells with striated cuticular 

 borders. Cilia are found on the tentacles and eyes of Nautilus. The 

 underlying connective tissue contains near its surface a layer of chromato- 

 phores, and of cells containing iridescent rods. The former are under the 

 control of the nervous system, the centre appearing to lie in the optic 

 ganglia. The chromatophore consists essentially of a cell charged with 

 pigment granules either scarlet, yellow, blue, or brown, one colour in each 

 cell. By the expansion of the cell the pigment is diffused, by its contraction 

 concentrated. The cell has been described as lying in a space traversed 

 by radial fibres which are attached to the cell-membrane, and supposed 

 by various authorities to be muscular, nervous, or composed of connective 

 tissue. See the original authorities, infra. The arms, siphon, and the 

 mantle fold forming the walls of the branchial cavity are muscular. The 

 mantle is contractile, the contractions expelling the water from the 

 branchial cavity for purposes of respiration, and when forcible subserving 

 locomotion as well. Masses of a peculiar cartilage are formed in the body- 

 walls, supporting the nervous system, the siphon in Nautilus, the bases of the 

 fins in Sepia ; and in all Decapoda on the dorsal aspect of the neck as nuchal 

 cartilages, and on the opposing points of the mantle surface as dorsal 

 cartilages, the latter present also in Octopoda ; and as * siphon-hinge ' 

 cartilages or sockets at the sides of the siphon in Decapoda which receive 

 fleshy processes developed on the mantle surface. Pores variable in 

 number exist on the back of the head, or at the bases of the arms, leading 

 into sub-integumental sacs of unknown function. A sac exists at the 

 base of each long arm in Decapoda ; and in Loligo the arm may be par- 

 tially, in Sepia, Sepiola, and Rossia completely, coiled within it. 



The nervous system consists of three pairs of ganglia concentrated 

 round the oesophagus a cerebral, a pedal, and a pleuro-visceral. They 

 have a band-like form in Nautilus, and are not well differentiated from the 

 connectives. The cerebral ganglia are connected with buccal ganglia in 

 the Dibranchiata, a single pair in Octopus, a double in Decapoda. The 

 pedal ganglia supply the arms and lobes of the fore-foot, as well as the 

 siphon. The pleuro-visceral give off a large number of nerves in Nautilus, 

 to the mantle, branchiae, and genitalia ; in Dibranchiata right and left 

 pleural and visceral nerves to the mantle, to the branchiae and viscera 

 respectively. Each pleural nerve ends in a ganglion stellatum, from which 

 nerves radiate into the mantle. The Dibranchiata also possess a well- 

 developed enteric system of nerves connected with the buccal ganglia and 



