820 



LIFE : OUTLINES OF GENERAL BIOLOGY 



gressive lines are to be found to-day, especially among sedentary 

 forms, such as barnacles and Tunicates, among easygoing forms, 

 such as Tardigrades and blindworms, and among parasitic forms, 

 such as tapeworms and lice. Similarly in the geological record ; among 

 types preservable as fossils there are many instances of reversed 



Fig. 139. 



An Evolution Series of Nautiloid Cephalopods. After Lull. I, a straight- 

 chambered Orthoceras type; II, a slightly curved Cyrtoceras type; 

 III, a more curved Gyroceras type; IV, the Nautilus type, the only 

 survivor in this series. 



evolution, as when some of the spirally coiled Cephalopods, on both 

 Nautiloid and Ammonoid lines of evolution, are succeeded by related 

 forms which are loosely coiled, and these again by slightly curved or 

 even straight forms, which have returned to the oldest and more 

 primitive type of shell. 



(c) It is necessary to make a distinction between increasing 



