THE CYSTIDEA 43 



it. Thus is evolved a highly specialised type of stereom-folding 

 known as a " pectini-rhomb." Now, although it is difficult to 

 separate all the forms at the first parting of the ways, it is soon 

 seen that diplopores are almost confined to the genera with 

 epithecal extensions of the subvective system, while those with only 

 exothecal extensions are characterised by pore-rhombs or pectini- 

 rhombs. There is therefore justification for the old divisions 

 DIPLOPORITA and RHOMBIFERA, as orders, in a restricted sense. 



The Diploporita (p. 70) show a gradually increasing regularity 

 of structure in the food-grooves, and in their relations to the 

 theca, leading almost imperceptibly to the Blastoidea. So much 

 is this the case that it seems well to separate from the Cystidea 

 certain forms in which " the radial polymeric symmetry " is " in 

 complete correlation with the radial symmetry of the ambulacra " 

 (see definition, p. 39), and to refer them to the Blastoids as an 

 order Protoblastoidea (p. 79). The only alternative is to make 

 the Blastoids an order of the Cystidea. 



In many of the Rhombifera (p. 52) a peculiar modification of 

 the food-grooves takes place, in that they are continued over the 

 theca, not directly on the thecal plates themselves, but by a 

 proliferation of plates from the mouth region. The grooves thus 

 formed have been termed " recumbent arms " or " pseudambulacra," 

 and are fringed with brachioles. This type of ambulacral structure 

 was independently developed in this order more than once ; but it 

 is most common in the group of genera characterised by pectini- 

 rhombs and by peritamerism in the theca (family Glyptocystidae, 

 p. 58). A group with pore-rhombs highly developed inside the 

 theca, and with hexamerous symmetry, is distinguished as the 

 family Caryocrinidae (p. 65). In it the food-grooves tend to be 

 enclosed by thecal plates (" hypothecal "). 



The orders already mentioned do not include all genera that 

 come under the terms of our definition of Cystidea. From early 

 forms of Rhombiferi, or perhaps even directly from Amphoridea, 

 there arose a small group in which neither diplopores nor pore- 

 rhombs were developed, at all events to the same extent, but the 

 number of thecal plates was greatly lessened and exothecal 

 brachioles were developed. The best known of these is Cryptocrinus 

 (p. 69). One might adopt APORITA (sens, sir.) as an ordinal name. 



ORDER 1. Amphoridea, Haeckel (1896, pars). 



Primitive Cystidea in which radial symmetry has affected 

 neither food-grooves, nor thecal plates, nor, probably, nerves, 

 ambulacral vessels, nor gonads. 



Haeckel included under this name rather more forms than are 

 here referred to it, and separated them from the Cystidea as a 



