3°4 



MANUAL OF ZOOLOGY 



surface, which contract and expand under nervous influence 

 (Fig. 189). 



On the postero-ventral aspect of the body the mantle 

 encloses a wide cavity, the mantle-cavity (Figs. 190, 191), 

 in which the ctenidia are lodged, and on the wall of which 

 are situated the anal, excretory, and reproductive apertures. 

 The mantle-cavity communicates at its oral end by a wide 

 slit with the exterior; but this is capable of being closed, 



rad-.rmis 



Fig. 189. — Chromatophore c-t* Sepia, magnified, nuc, nuclei in wall of sac; pigm, 

 pigment ; rad. 71171s, vadiating strands of muscle. ( After Vogt and Jung.) 



so that, when the walls of the cavity contract, a stream of 

 water is ejected through the funnel, and the animal is 

 propelled in the aboral direction. Swimming is also effected 

 in the Dibranchiata by means of a pair of fins in the shape 

 of muscular, lateral flaps. The ctenidia (cteri) are plume- 

 like, and are either two {Dibranchiata) or four {Tetra- 

 branchiata) in number. 



The mouth is provided with a pair of horny or calcified 

 jaws (Fig. 192, B) similar in shape to the jaws of a parrot. 

 The buccal cavity contains an odontophore. Opening into 



