THE EVOLUTION THEORY 



131 



In their development the Crinoids pass two distinct larval 

 stages. First the ovum produces a minute, free-swimming form, 

 resembling a barrel bound with five ciliated hoops. These and 

 a wisp of cilia at the most anterior end effect 

 locomotion. But soon the larvae give up 

 their free life, anchor themselves by means 

 of a ventral groove to Algse or a stone, and 

 change into a stalked larval form, entering 

 the so-called Pentacrinus stage. From this 

 proceeds finally the adult Echinoderm which 

 once more returns to a free life (fig. 41). 

 The appearance of a typical fixed stage in 



FIG. 40. 



the evolution of a free-living animal is 



FOBNABIA-LABVA OF A 

 MAKINE-WOBM, Bdla- 



noglossus. (After E. 

 Metchnikoff.) 



remarkable. Does not perhaps the bio- 

 genetic law apply here, and is not the free 

 mode of life in Antedon a recent 

 requisition ? Turning back the leaves 

 of the earth's history we find that 

 the Crinoids are a very ancient and 

 widely distributed group, and that 

 the species existing to-day are evi- 

 dently only the remainder of a vast 

 class which is rapidly approaching 

 extinction. The Palaeozoic period 

 represents the zenith of their develop- 

 ment, and though the various species 

 of Crinoids occurred in narrowly 

 defined regions, they are found in 

 such masses that frequently deposits 

 of a thickness of several metres 

 almost exclusively consist of their 

 limy armour. If we examine these 

 extinct forms accurately, we see that 

 they were in fact stalked, fixed 

 forms. Such forms still exist to-day 

 on the ocean bed, where conditions 



remain eternally alike the Ehizocrinus and Pentacrinus, which 

 I have already mentioned. 



Larval forms similar to those with which we met in the 



FIG. 41. LABVAL FOBM OF A 

 CBINOID. 



(1) Free-swimming larva, with 

 skeleton of adult forming in- 

 side ; (2) fixed stalked young 

 Pentacrinus stage. (After 

 Clauss.) 



