284 ZOOLOGY. 



nerwus ganglia much concentrated and protected by an imperfect cartila- 

 ginous capsule ; pharynx armed with two teeth like a parrot's beak, be- 

 sides an odontophore. Sexes distinct. Usually development is direct, 

 with no metamorphosis ; segmentation of the yolk partial, and a primi^ 

 tive streak is present as in birds and reptiles. 



Order 1. Tetrabranchiata. With four gills. (Nautilus, living ; Or- 

 thoceras, Goniatites, Ammonites, extinct.) 



Order 2. Dibranchiata. With two gills. (Spirula, Belemnites, ex- 

 tinct, Sepia, Architeuthis, Loligo, Ommastrephes, Octopus 

 Argonauta.) 



TABULAR VIEW OP THE CLASSES OP MOLLUSCA. 



Cephalopoda. 



Cephalophora. 



Lamellibranchiata. 



MOLLUSCA. 



Laboratory Work. The cuttles are not easy to dissect. A horizon- 

 tal section through the head will show the relations of the cartilaginous 

 capsule to the brain, optic nerves and eyes. The nervous ganglia can 

 only be traced after tedious dissection. To study the viscera freshly- 

 killed specimens are quite essential. 



