214 SYSTEMATIC ZOOLOGY. 



17. The Acephala may be divided into Asiphonida and 

 Siphonata. 



18. The CEPHALOPODA have no true foot, this being 

 represented by the siphon and the tentacles around the 

 mouth. 



19. The shell, when present, is unpaired. When present 

 it may be external or internal. 



20. The ink-sac is peculiar to Cephalopoda. 



21. The eyes are usually highly organized, and resemble 

 those of vertebrates. 



22. There are present special branchial hearts to force 

 the blood to the gills, and a systemic heart to force it 

 through the body. 



23. The Cephalopoda are divided into Dibranchiata and 

 Tetrabranchiata according to the number of gills. 



