102 



. THE LIVING WORLD. 



were, on the other hand, by far the most abundant of the echino- 

 derms. With the beginning of the Silurian (2) great numbers and 

 varieties were found in existence. The class was then divided into 

 three orders (Brachiata, Blastoidea, Cistoidea), ail of which reached a 

 high degree of expansion in the Paleozoic (2-4). After thus reaching 

 a culmination they gradually become less numerous and continued 

 to diminish until to-day, when there exists only the small number 



D 



FIG. io. Diagram illustrating the history of the Echinodermata. The letters 

 on the left indicate the different geological ages. E Echinoidea. O Ophiuroidea, 

 A Asteroidea. H Holothuroidea. C Crinoidea. B Blastoidea. Cy Cystoidea. 



above mentioned. Two of the former orders disappeared com- 

 pletely, and thus these that are left are simply surviving fragments of 

 a once predominant type. The modern crinoids, which are in some 

 respects different from the older ones, made their appearance in the 

 Triassic (6). 



Asteroidea (star-fishes). This class was also in existence in Si- 

 lurian (2) times. The type of star-fish found in these rocks is slightly 



