236 



HIGH SCHOOL ZOOLOGY. 



cavity, and thus to be employed for swimming. Horny jaws 

 bound tlie aperture of the mouth ; and the nervous system and 

 sense-organs, which are protected by a cartilaginous endoskele- 

 ton, attain a degree of development not to be met with else- 

 whex-e among the Invertebrates. These foi-ms have two gills 



Fi^. 162.— «, aCephalopod (ioH(/o); 6, a Scaphopod {Bcntalium) ; c, a Pteroport 

 {Eytdea), 



in, mantle-margin of Loligo ; /, the funnel ; /', the fins of Hyalea, between them the 

 mouth ; jhi, processes of the mantle-margin. 



(Dibranchiata) in the mantle-cavity, but the Pearly Nautilus 

 (y. 2>ompUio) has four (it is a Tetrabraachiate form), and it 

 further differs from the cuttle-fish in the number of the oral 

 subdivisions of the foot, in the presence of an external chambei'ed 

 shell, and the absence of the ink-bag, (a very characteristic 

 organ of defence of the Dibranchiata, which furnishes the 



