XII 



PHYLUM MOLLUSCA 



749 



The edges of the mantle grow out into prominent folds to form 

 the mantle-cavity, into which the gills are drawn. Lateral out- 

 growths have already formed the rudiments of the fins. The 

 arms grow out into more and more prominent processes on which 

 the suckers become developed, the second pair the prehensile 

 arms (ar. 2) soon becoming distinguishable from the rest by its 

 greater length. 



As the embryo increases in size, the yolk becomes gradually 

 absorbed, and the yolk-sac decreases in bulk, until, when the 

 embryo leaves the egg, it has almost completely disappeared. 



FIG. 662. Two late stages in the development of lioligo, seen from the funnel side. Letters as 

 in preceding figures. (After Korschelt and Heider.) 



Distribution. The Cephalopoda are all marine, and range from 

 tidal limits to a considerable depth. A large number are pelagic. 

 They are, nearly without exception, carnivorous. In length they 

 range from an inch or two to as much as fifty feet the gigantic 

 members of the group such as Architeuthis, being by a long way 

 the largest of invertebrate animals. Like the other classes of 

 Mollusca they are most abundant in tropical and warm temperate 

 seas. 



If the Ammonites are to be included among the Tetrabran- 

 chiata, that sub-class was most abundantly represented during 



