V. CEPHALOPODA 



and Japan. One of the Newfoundland specimens had a body twenty 

 feet long from head to tail, and one of the arms was thirty-five feet in 

 length. 



The body of a cephalopod is divided by a constriction into head and 

 trunk. At the extremity of the head is the mouth, and around this a 

 circle of arms or tentacles. Each tentacle is tapering and bears on its 

 oral surface rows of suckers (in some species altered to hooks). The 

 Octopoda have eight of these arms, all equal in size (fig. 347), four on the 

 right side, four on the left. The Decapoda (fig. 348) have in addition two 

 longer arms between the third and fourth of the Octopoda, counting from 

 the dorsal side. These Dear suckers only on the enlarged tips and can be 

 retracted into special pouches. 



ae 



Tnf 



JV.Qp 



FIG. 349. FIG. 350. 



FIG. 349 Diagrammatic section of Cephalopod eye (after Gegenbaur). ae, 

 argentea (chorioid); C, cornea; ci, ciliary process; go, optic ganglion; ik, iris; k, carti- 

 lages; L, lens; p, pigment layer; Re, cellular layer of retina; Ri, rod layer of retina; 

 u 1 , white body. 



FIG. 350. Schematic section of eye of Nautilus (from Balfour). .1, aperture <>f 

 optic cup; hit, iris-like fold of integument, N op, optic nerve: R, retina. 



Behind the tentacles are the pair of large eyes which superficially 

 closely resemble those of the vertebrates, since they have a transparent 

 cornea and a large pupil surrounded by an iris. Internally the resem- 

 blance is not less pronounced (fig. 349). Behind the iris is a lens and a 

 vitreous body, the latter being bounded by the retina and this in turn by a 

 pigmented silvery layer, the argentea or chorioid, which contains cartilages 



