MOLLUSC A 251 



A B 



FIG. 67. An octopus. A, upper surface showing body and eyes, B, lower surface 



showing the adhesive disks. 



footed) from the eight long tentacles, which bear ad- 

 hesive disks. The eyes are often exceedingly large and The eyes 

 well developed, superficially extremely like those of 

 vertebrates, and possessing similar parts. Since the 

 mollusks belong to quite a different stem from the 

 vertebrates, the independent development of eyes so 

 similar in form is a remarkable example of "convergent 

 evolution." Most cephalopods (but not the Nautilus] The ink sac 

 have an ink sac, from which is expelled a black sub- 

 stance serving to confuse an enemy and facilitate suc- 

 cessful flight. In function it corresponds with the 

 "smoke screen" used by steamers as a protection 

 against submarines. It is this black material which 

 has been used as a paint under the name of "sepia." 



The shell may be absent, as in the Octopus. When sheiiofthe 

 present, it may be external, as in the Nautilus, or in- nautllui ? and 



Ammonite 



ternal. In the latter case it may be quite rudimentary, 

 a condition paralleling that of the slugs. The nautilus 

 and ammonite shells are divided internally by septa into 

 a series of compartments, only the last or outer of 

 which is occupied by the animal. In former geological 

 ages the Cephalopoda were more abundant and varied 



