6oo 



ECHINODERMATA BLASTOIDEA 



chap, xx 



by the extension of the side-plates of the radial vessel, and the 

 whole group of folds has a common opening near the mouth ; 

 indeed, in the highest form there is one common " spiracle " for 

 the two groups of folds in an interradius, which in one interradius 

 is confluent with the anus. The hydrospires, when they reach 



this form, irresistibly recall the 

 genital bursae of Ophiuroidea 

 (Fig. 214, p. 490), and very 

 possibly served the same purpose. 

 Eeviewing the whole group of 

 the Pelmatozoa, we see that in 

 the Cambrian they begin with 

 the extremely primitive Thecoidea 

 and Carpoidea, together with 

 some obscure forms which, com- 

 bining a stem with pentamerous 

 symmetry in the calyx, are sup- 

 posed to be the forerunners of the 

 Crinoidea. In the Lower Silurian 

 or Ordovician the two groups of 



Fig. 279. Granatocrinus norwoodi. A, . , s-i j i i ,, . 



view of whole animal ; B, section of the Cystoidea make their appear- 

 radius ; c, an isolated finger, hyd, ance possibly independently de- 



Hydrospire ; I, lancet plate ; pinn, . 



finger ; p. P , covering plate ; R and veloped from either Carpoidea or 

 D .both signify radial plate. (After pr i m itive Crinoidea, which in 



Zittel.) r ' 



this period are present in unmis- 

 takable form. In the Upper Silurian the Blastoidea appear, 

 distinguishable from the most regular Cystoidea only by their 

 hydrospires. It seems practically certain that they were 

 developed from Cystoidea, and we follow Jaekel in believing 

 that they arose from Dichoporita. The Carpoidea do not 

 extend beyond the Ordovician, and by the end of the Carbo- 

 niferous period Cystoidea and Blastoidea die out, leaving only 

 the Crinoidea, which at that period were at their maximum 

 development. From the Carboniferous to the present day the 

 Crinoidea have continually decreased, leaving in recent seas, as 

 sole representatives of the Pelmatozoa, only the few forms 

 described at the beginning of this chapter. 



