598 



ECHINODERMATA CYSTOIDEA 



the tNvo foregoing classes. rormeiiy this class was a lumber- 

 room in which were placed all primitive irregular Pelmatozoa. 

 The labours of Jaekel ^ have, however , dispelled the mist which 

 enveloped this group, and in his monograph all that can be 

 extracted both from superficial examination and dissection of 

 these fossils is contained. It seems possible to the present 

 author that the class may eventually require to be divided into 

 two, corresponding to the two main divisions which Jaekel 

 recognises, viz. Dichoporita, with pectinated rhombs, and 

 DiPLOPORiTA, with diplopores. 



The pore-rhombs of the Dichoporita (indicated in Fig. 277 

 by the small parallel lines crossing the boundaries of the plates)^ 



mouth 



amb. 



Fig. 277. — Echinospliaerites aurantium. A, from above ; B, from the side ; C, neigh- 

 bourhood of mouth, enlarged, amh, Anibulacral groove with side-plates and cover- 

 ing plate ; mad, madi-eporite. The short parallel lines across the sutures are the 

 " pore-rhombs. " (After Zittel. ) 



were, according to Jaekel, nothing but a series of folds of thin 

 integument projecting into the interior, the outer opening of which 

 in most cases adhered in the middle, leaving two pores connected 

 by a groove. The inner boundaries of the folds are sometimes 

 preserved, but in many cases they were entirely devoid of calci- 

 fication, and so were lost. The radial vessels either branched a 

 great deal, giving rise to a multitude of fingers, or, as in Echino- 

 sphaerites (Fig. 277), there were a few long fingers supporting a 

 reduced number of radial canals. In some cases the calyx can 

 be analysed into a regular series of cycles of plates, consisting of 

 basals, orals, and three intervening whorls, thus including one 

 more ring than the calyx of Crinoidea. Jaekel regards this as 

 a primitive arrangement, believing that the irregularity seen in 

 Echinosphaerites is secondary. This is a doubtful hypothesis. 



^ Stammesgeschichte der Pelmatozoen, pt. i. 1S99. 



