MANUAL OF ZOOLOGY 



SECT. 



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 (cfen.) are plume- 

 like, and are either two 

 (Dibranchiata) or four 

 (Tetrabranchiata) in 

 number. 



The mouth is pro- 

 vided with a pair of 

 horny or calcined jaws 

 (Fig. 1 80) similar in 

 shape to the jaws of a 

 Parrot. The buccal 

 cavity contains an odon- 

 tophore. Opening into 

 the terminal part of the 

 intestine close to the 

 anal aperture is the duct 

 of a peculiar gland the 

 ink-gland (Fig. 181, /'.). 

 This secretes a black 

 substance, the ink^ 

 which is discharged 

 when the animal is irri- 

 tated or alarmed, and, 

 mingling with the water 

 in the mantle cavity, 

 is ejected as a dark 



cloud, under cover of which the animal may elude the 

 pursuit of an enemy. 



The heart and vascular system reach a high stage of 

 development. The heart consists of a median ventricle and 

 two (Dibranchiata) or four (Tetrabranchiata) elongated lateral 



B 



FIG. 180. Sepia officinalis, jaws. A, in 

 situ; B, removed and slightly enlarged. 

 (From the Cambridge Natural History.} 



