130 THE LIVING WORLD. 



Cephalopoda (squids), of an occasional order among 

 the rest of the mollusks, of our common lobsters, 

 shrimps, and crabs, of the order of modern sea- 

 urchins, and seemingly so of some of the orders of 

 Coelentera (Alcyonaria), etc., though of this we can- 

 not be sure, since the animals appear to be so poorly 

 adapted to preservation that their early history is 

 uncertain. 



It is plain that all of the orders of animals existing 

 in great abundance to-day must come under one of 

 the two classes last mentioned, and it will thus follow 

 that nearly all of the animals well known to the gen- 

 eral reader have had the history of constant expan- 

 sion from the time of their appearance until to-day. 



Fig. 20. Not infrequently in the history of some 

 groups there has occurred a long period of stationary 

 condition, followed by a rapid development (see Fig. 

 20). A very striking illustration of this is shown in 

 Fig. 10, which represents the history of the echino- 

 derms. From the figure it will be seen that the sea- 

 urchins appeared in the early Silurian (2), but during 

 this and the long Paleozoic ages remained practically 

 stationary. With the beginning of the Triassic (6), 

 however, some influence caused the urchins suddenly 

 to begin to develop into new forms. In a very short 

 time, comparatively, there were then produced a 

 large number of sub-orders and families, and the sea- 

 urchins were brought into existence practically as 

 we have them in the world to-day. By the time of 

 the Cretaceous (8) this development had ceased, and 

 from that time the echinoid group, as well as the rest 

 of the echinoderrns, has remained stationary, or per- 



