334 



ing genera .... shows that in general the older genera have the 

 more tubular, simpler thecal, with the less protected apertural margins. It 

 is, hence, apparent that the stolonal or earlier thecse of the rhabdosomes 

 represent indeed the older types of thecal form." (48.) 



Other Glasses. — The case of the larva of Antedon has already been 

 referred to. As pointed out by Bather (1), the stem ossicles of the larval 

 Antedon are of a complex and specialized type, and in a general way re- 

 semble the stem ossicles of the Bourgueticrinidse of the Upper Cretaceous. 

 It is held by Bather that the structures of the adult ancestors have been 

 pushed back by acceleration to the larval stages of the existing Antedon. 



Recapitulation is also shown in the anal plate of Antedon. The anal 

 plate appears between two of the radials and on the same level with them. 

 Subsequently it is lifted out from between the radials, and the latter close 

 beneath it. Still later the anal plate is resorbed entirely. That this is the 

 recapitulation of an adult character and not of a larval character, as con- 

 tended by Hurst, is shown by the fact that the oldest crinoids do not possess 

 the anal plate at all. It appears from paleontological evidence that this 

 plate first appeared above the level of the radials, that it gradually sank 

 down between the two posterior radials, and that at a far later period 

 (at about the close of the Paleozoic) it gradually passed upward again as 

 it does in Antedon, and eventually disappeared. 



Jackson has shown that there is good evidence of recapitulation among 

 the fossil echinoids (33). In most regions of the echinoid the develop- 

 ment is obscured by the more or less extensive resorption, but the plates 

 of the corona may show by their position and number, the course of devel- 

 opment. Jackson holds that the introduction of columns of plates, both 

 interambulacral, and ambulacral, in Melonites, etc., indicates the stages of 

 growth through which the individual has passed in its development. . He 

 shows that two columns of ambulacral plates "may be accepted as the 

 usual characteristic of the whole class, which finds its representative in 

 the majority of the adults, in nearly all young, and in the adult of the 

 simplest and oldest known type, Bothriocidaris." 



Interambulacral areas originate ventrally in a single plate. Only one 

 genus is known, however, that has a single row of plates in the adult, 

 namely Bothriocidaris. This is the simplest known and "perhaps the 

 simplest conceivable echinoid." 



