74 THE ORIGIN OF MAN 



external shells, and because they had but two in- 

 ternal gill-plumes, the name Dibranchiata has 

 been given to them. They had ten arms, pos- 

 sibly eight short and two long protrusible ones. 

 In the Mesozoic these arms were often provided 

 with bent chitinous hooks for holding, and more 

 rarely with holding suckers. They are all pro- 

 vided with an internal ink-sac containing sepia, 

 a brown-black fluid that mixes readily with 

 water; this the animals squirt in front of them 

 when in danger and then make their escape back- 

 ward away from the defensive screen of colored 

 water. Like the other cephalopods, they are pro- 

 vided with a siphon through which water is shot 

 forward thus propelling the animal forward. 

 They can also swim with the aid of their side 

 fins. 



In the Comanchian period, the reptiles prob- 

 ably attained the zenith of differentiation, when 

 the dinosaurs were present, not only in the great- 

 est variety, but in the greatest size as well. The 



