648 



EVOLUTION OF ANIMALS 



Part V 



TinivcMs 



Dorse/ /obe 

 or manHe 



Siphon 



Sfie/t mt/scfe Marrf/e 



Siphuncle 



between chambers 



Fig, 31.15. Cephalopods, the swiftest of all mollusks. Upper left, sepia, the 

 cuttlefish with one tentacle stretched forward gripping a crab with its vacuum 

 disks. The white "cuttlebone" fed to canaries for lime is the shell of the cuttle- 

 fish. The name sepia is due to the brown inklike secretion that the cuttlefish throws 

 off when disturbed. Upper right, paper sailor (Argonauta). Female in floating 

 position. Paper sailors float on the surface of the warmer waters of the Atlantic and 

 Pacific oceans, occasionally in coastal waters. The thin papery shell is secreted by 

 the flattened arms. It is not attached to the body, has no partitions and is mainly a 

 carrier for the eggs. The female is eight inches long, the male about one inch long. 

 Lower, chambered nautilus (Nautilus) cut open to show the successive chambers 

 that have been occupied as the animal has grown. A cord of living tissue extends 

 from the animal's body to the first chamber that it occupied. {Upper left after 

 Boulenger. Upper right after Claus and Sedgwick. Lower after Ludwig and Leunis. 



of fishes. They are themselves in turn the prey of fishes, but they are swift 

 dodgers. They are actually jet propelled, darting with sudden speed when 

 water gathered in the mantle cavity is spurted out of the siphon with great 

 force (Fig. 31.16). They swim by undulating movements of the fins, actually 

 flaps of the mantle, not at all like the fins of fishes. 



Structure. Squids have 10 arms, including one pair with grasping tentacles 

 much longer than the others. When a squid is swimming it holds the arms 

 close together and uses them as rudders for steering. A squid darts at its prey 

 arms foremost and when almost upon it spreads them like the rays of a daisy, 

 stretches out the tentacles, grips the prey, pulls it back against the sharp beak 

 in the meantime clasping it with the other arms. Next to the arms, the eyes are 

 the most prominent features of the head. Although entirely different in their 

 development, they are the camera type like those of vertebrates. The squid is 

 an example of the association of the active hunting habits of a carnivore 



