304 MANUAL OF ZOOLOGY SECT. 



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, 



771 



FIG. 189. Chromatophore of Sepia, magnified, nuc, nuclei in wall of sac ; pignt, 

 pigment ; rad. mus, radiating 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 (cten} 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 



