CEPHALOPODA. 



1041 



ff 



and section of one of the optic nerves puts an end to voluntary changes 

 of colour on the same side of the body. The chromatophores are 

 chiefly distributed over the anterior surface (or upper surface when 

 the animal is in its natural position) of the mantle, the head, and the 

 external sides of the arms. In the Decapoda there is, in addition to 

 the chromatophores, a layer of reflecting cells which give these animals 

 their iridescent hues. 



The digestive tube of the Cephalopoda comprises a buccal mass with 

 2 mandibles and a radula, a long oesophagus, a muscular stomach with 

 pyloric caecum, and a short in- 

 testine which turns forward and 

 opens in the middle line below 

 the funnel. 



The buccal aperture, situated 

 in the middle of the pedal ap- 

 pendages, is surrounded by a 

 circular lip garnished with pa- 

 pillae. Furthermore, in the de- 

 capodous Dibranchia there is a 

 buccal membrane which may be 

 very extensive and be divided 

 into lobes alternating with the 

 arms, and the lobes may even 

 be furnished with small suckers, 

 as may be seen in some species 

 of Loligo. 



The buccal cavity, or pha- 

 rynx, has very thick muscular 

 walls. Internally it is provided 

 with 2 powerful mandibles, one 

 ventral and the other dorsal ; 

 the tip of the ventral mandible 

 overhangs that of the dorsal, 

 forming a beak like that of a 

 parrot. These mandibles have 

 recurved insertion plates, to 

 which the large muscles form- 



777 



ing the greater part, of the 

 mass of the buccal bulb are at- 

 tached. 



As in the Amphineura, the 

 Gastropoda, &c., the floor of the 

 buccal cavity is occupied by the 

 anterior part of the radula, which 



DIAGRAM OF THE .STRUCTURE OF A 

 CEPHALOPOD, AS SEEN FROM THE 

 LEFT SIDE IN ITS PROPER MORPHO- 

 LOGICAL POSITION. 



a, Anus ; ar, arms ; ce, central nervous 

 system, with eye ; COR, ccelom ; fu, 

 funnel ; g, gill ; gg, gonad ; ;/o, gona- 

 duct ; h, heart ; k, kidney ; I, liver ; 

 wt, mouth ; pa, mantle ; re, renal 

 orifice ; rp, reno-pericardial orifice ; 

 st, stomach. 



(After Pelseneer.) 



issues from a pharyngeal caecum. 



Each transverse series of this radula is formed by a median tooth, 



with 3 symmetrically disposed teeth on either side ; the only excep- 



