104U CEPHALOPODA. 



well developed, hut highly specialized to form a funnel. Through 

 this tube tin- excrements, th'e secretion of the ink-sac, and the genera- 

 tive products are ejected. The interior of the funnel is generally 

 provided with a larger or smaller valve, attached to its anterior or 

 dorsal face. 



In the Tetrabranchia the mantle is covered by an external shell. 



/ 



The female Aryonauta also bears an external shell which covers the 

 mantle, but has no muscular attachments and is not homologous with 

 the shells of other Cephalopoda : it does not originate from a pre- 

 conchylian invagination or shell-gland, but is of pedal origin, and is 

 only formed some ten or twelve days after birth by the palmate 

 extremities of the two dorsal arms. The animal is not attached to 

 this shell. In all other Cephalopoda the shell is covered over by the 

 mantle, or at least is partly covered in Spirula. The shell, therefore, 

 is internal, and often is rudimentary, as in the majority of Decapoda, 

 or it may be nearly obsolete, as in the Octopoda. In all living 

 Cephalopods except Nautilus the shell is localized on the anterior 

 or physiologically dorsal side of the body, and is enclosed by the 

 mantle, which therefore appears to be a naked bell-shaped sac. 



In the Dibranchia the mantle is a very muscular organ, which, 

 by its contraction, serves two purposes : by alternately and rhyth- 

 mically drawing in and forcing out the water that enters the pallia! 

 cavity between the funnel and the border of the mantle it acts as an 

 accessory respiratory organ ; and by violently expelling water through 

 the funnel it acts as an efficacious locomotory organ, causing the 

 animal to execute sudden retrograde movements. 



In the majority of Cephalopods with internal shells (Decapoda) 

 and in the Cirrhoteuthidce the mantle is produced into lateral sym- 

 metrical expansion or fins of various form and position. These organs 

 always originate at the aboral extremity of the mantle as two triangular 

 or rounded outgrowths. 



Beneath the epithelium the integument contains, at least in the 

 Dibranchia, chromatophores, or extensible pigment-cells, whose activity 

 produces the remarkable colour-changes characteristic of these animals. 

 The chromatophores are cells originally of ectodermic origin, which 

 sink below the epipthelium and become connected with contractile 

 radiating mesodermic fibres. The pigment-cells are simple, but multi- 

 nuclear, since they contain secondary nuclei situated at the bases of 

 the muscular fibres. Different cells contain different-coloured pig- 

 ment : yellow, brown, red, or blue in the Decapoda. Each cell ex- 

 hibits a constant though feeble tremulous movement, and may suddenly 

 be extended, by a reflex action, under the influence of emotion or 

 excitation, or as a more direct result of volition the chromatophores 

 of the same colour may assume a definite condition of contraction or 

 expansion, which gives the body a tint analogous to that of surround- 

 ing objects. In the latter case the action of the chromatophores is 

 under the direct influence of the cerebral centres of the nervous system, 



