THE BLASTOIDEA 



79 



Edrioasteroidea they are separated by the presence of brachioles and 

 absence of ambulacral pores. Their line of evolution, though in 

 some respects parallel to those of the Callocystinae and Edrioas- 

 teroidea, was independently derived through the Diploporita from 

 the primitive Amphoridea. 



Within the class itself may be traced the increase of penta- 

 merism, combined with a lessening in number of the thecal plates 

 until a very definite arrangement is reached. At the same time, 

 there is a concentration of semi-porous structures into the inter- 



Fio. I. 



AsteroUastus. 1, oral surface of A. stellatus; not quite correct, as anal area is not shown. 

 2, side view of A. Volborthi. Both figures adapted from Schmidt, and x 2 diain. ami, so- 

 called ambulacrum; br, brachioles, supported on W, brachiole-facets ; f.g, food-groove; pp, 

 pore-plate, at adoral end of which is deltoid ; R, radial, at end of ambulacrum ; St, stem. 



ambulacral region, and the evolution therefrom of elaborate 

 respiratory organs (hydrospires) ; this is probably connected with 

 the fact that all external connection with the water-ring (by means 

 of a madreporite or hydropore) seems to have disappeared in the 

 more specialised forms, and that, if the negative results of research 

 may be trusted, there were no extensions of the water-vascular 

 system into the brachioles. 



A somewhat arbitrary line may be drawn below forms that 

 have acquired the normal definite number of plates and the 

 hydrospire- folds hanging far into the thecal cavity; and while 

 such forms constitute a grade Eublastoidea or Blastoidea sensu 

 stricto, those below the line are Protoblastoidea. 



GRADE A. Protoblastoidea, Bather (1899). 



Blastoidea without interambulacral groups of hydrospire-folds 

 hanging into the thecal cavity. 



