THE BIOGENETIC LAW 423 



Ccelenterata. — It has been shown by Dr. C. E. Beecher that 

 the young stages of the Favositidse correspond to Aulopora, or 

 to some other similar unspecialized genus. This same conclu- 

 sion has been reached by Dr. G. H. Girty based on a study of 

 the ontogeny of Favosites, Syringopora, and other tabulate corals, 

 all of which are shown to go through an Aulopora stage of 

 growth. 



" Echiiwdermata. — The only crinoid of which the ontogeny is 

 known is Ante don, which has been shown by Sir Wyville Thom- 

 son to go through successively stages corresponding to the 

 Iclithyocrinoidea of the Paleozoic, and Pe?itacrinus of the Mesozoic, 

 before it becomes free swimming and takes on the characters of 

 Afitedon. 



Dr. R. T. Jackson has been able to prove even in the Paleo- 

 zoic sea-urchins the possibility of correlating growth stages with 

 phylogeny, in spite of the great difficulties due to resorption of 

 plates, and change of form. 



Brachiopoda. — According to Beecher all brachiopods go 

 through a primitive protegulum stage, correlative with the sup- 

 posed ancestor of the class, although Paterina, which was for- 

 merly supposed to be this radicle, has been shown to be much 

 more highly specialized than the protegulum stage. The later 

 stages of growth of this class are capable of even more remark- 

 able correlation, as has been shown by Beecher in a number 

 of papers, where every stage of growth is distinctly homol- 

 ogous with well known pre-existing genera ; and these same 

 successive genera show a gradual transition in the adults. 



Even among the Paleozoic spire-bearers {Helicopegmata) , this 

 holds good, for Beecher and Schuchert have demonstrated that 

 the early stages of this group are homologous with the terebratu- 

 loids {Ancylobrachia) , aud more especially with the Paleozoic 

 genus Centronella, the most primitive of the loop-bearing brachio- 

 pods. 



Mollusca. — Jackson's correlations of the stages of growth of 

 pelecypods with their race history have already become classic ; 

 according to these, every pelecypod begins its bivalve state with 



