Chap. 31 



MOLLUSKS SPECIALISTS IN SECURITY 



647 



Fig. 31.14. Common octopus or devilfish, Octopus vulgaris. When they are ex- 

 tended the arms of this species may have a span of over six feet though they are 

 usually much shorter. The arms of an Octopus apollyn of the western coast of the 

 U. S. may have a span of 20 feet. (Photograph courtesy, Douglas P. Wilson, 

 Marine Biological Laboratory, Plymouth, England.) 



mauve and rose, and the yellows and browns that shift over the body of an 

 excited squid. Some species have bioluminescence to add to their beauty of 

 daytime color (Fig. 31.17). In the squids and sepias, an ink gland secretes a 

 dark fluid that is stored in the ink sac. When the owner is disturbed it shoots 

 jets of ink from the siphon, creating a cloud in the water that hides its escape. 

 Living cephalopods are a small group, but their ancestors once swarmed the 

 seas and fossils of some 10,000 different species are known. The pearly nauti- 

 lus (Nautilus pompilius) is the only living relic of great numbers of predeces- 

 sors which also had spiral shells, divided into compartments by septa. As a 

 nautilus grows, it enlarges its shell and secretes a partition behind it so that the 

 whole shell comes to be a series of chambers empty except for the cord of living 

 tissue connecting the body to the first small chamber (Fig. 31.15). Among the 

 ancestors of the pearly nautilus was one whose fossil shell is 1 5 feet long. The 

 shell of the living nautilus measures about 10 inches. 



The Squid — Loligo 



The common squid, Loligo pealii, of the Atlantic, is about 10 inches long; 

 that of the Pacific is a little more than half that. Squids range from those that 

 are less than two inches long to the giant squids of the deep sea some of them 

 probably having an over-all length of over 50 feet — by far the largest living 

 invertebrates. All of them are fierce carnivores that follow and attack schools 



