MOLLUSKS 165 



The male Argonaut is only about one-tenth as large as the 

 female, being about one inch in length. Previous to the breeding 

 season the third arm on the left side is seen to be developing inside 

 of a sac. Later this sac splits along on one side and turns inside out, 

 thus freeing the arm, which is then seen to be more than twice as 

 long as the ordinary arms of the animal, and to terminate in a long 

 pointed filament which was itself developed in a sack very much as 

 was the base of the arm. A number of long filamentous tubes con- 

 taining spermatozoa are placed within the cavity of the sac at the 

 base of the arm. At the breeding time the entire arm is cast ofF, 

 enters the mantle cavity of the female, and adheres to her l^ody. 

 The male Argonaut never develops a shell. 



It is in the contemplation of creatures such as this that we 

 come to realize the hopelessness of any attempt to measure by our 

 puny standards the immensity of time that has elapsed since evo- 

 lution began to mold the manifold forms of life. How long may it 

 have been before such a remarkable contrivance as the shell-like 

 brood-pouch and such a curiously modified arm as that of the male 

 Argonaut could have been developed? 



Altogether there is no more comprehensive picture of the 

 course of evolution than that furnished by the fossil shells of 

 Cephalopods. We see the straight-shelled Xautilus race that 

 swarmed in the ancient Silurian seas, when the whole western half 

 of New York State was submerged by an ocean continuous with 

 what is now the Pacific. Afterwards in Devonian times we find the 

 sculptured Ammonites appearing in a vast variety of forms. Then 

 the Nautilus race slowly faded away until to-day we find its last 

 lingering descendant living in the depths of the Pacific, while the 

 Ammonites, their shells coiling and uncoiling in writhing, snake- 

 like shapes, died out forever, while the Chalk cliffs of England were 

 yet beneath the sea. 



Only the soft bodied squids and octopi which first appear in 

 Triassic seas, still survive in reduced numbers in the oceans of 

 to-day. 



