CYSTOIDEA. 45 1 



cases (as in Caryocrinus and Hemicosmites) they are only developed 

 on the side-walls of the calyx, and are absent from its upper surface. 

 In other cases, again, as in Plenrocystites (fig. 326, b) or Callocystites 

 (fig. 322, c), the pore-rhombs, or "pectinated rhombs," as they have 

 been often termed, are comparatively few in number, and their 

 component halves not only stand on contiguous plates, but may be 

 separated externally by an interval. 



It would seem extremely probable that the differences in the form 

 and arrangement of the pores in different groups of Cystideans, 

 which have been pointed out above, are really an indication of a 

 fundamental difference in the nature and function of these structures. 

 In those Cystideans which possess " pore-rhombs " (Rhombiferi), 

 there seems no reason to doubt that these structures were connected 

 with internally-placed, folded tubes similar to the " hydrospires " of 

 the Blastoids, and that they acted as respiratory organs. On the 

 other hand, it has been suggested by Loven, with much probability, 

 that the linked pores of Glyptosphcerites and its allies (Diploporitidtz) 

 were connected with the ambulacral system, and that, like the pores 

 in the test of the Urchins, they were occupied by pedicels. On 

 this view, " hydrospires " are wanting in the types in question, and 

 the function of respiration would be discharged by the tube-feet. 

 If this suggestion be accepted, the Diploporous Cystoids would be 

 comparable to the Sporadiporous Holothurians or to such Urchins 

 as Melonites. They would, however, differ from the Urchins gener- 

 ally in the want of definite radial symmetry in the plates of the 

 test, and also in the fact that the pores for the tube-feet per- 

 forate the plates of the interambulacral areas, instead of being con- 

 fined to the ambulacral areas only. 1 The Cystoids {Aporitidcz) 

 in which neither pores nor " pore-rhombs " are developed, occupy 

 a dubious position from a systematic point of view. They differ 

 widely from the preceding groups in some respects, and it is pos- 

 sible that they may ultimately find a place elsewhere. 



As regards their distribution in time, the Cystideans are not only 

 wholly extinct, but they are entirely restricted to the Palaeozoic 

 period, ranging from the Cambrian period to the Carboniferous 

 Limestone, but attaining their maximum of development in deposits 

 of Ordovician age. The few Cambrian forms which have hitherto 

 been detected are only known from imperfect examples, and their 

 precise structure is incompletely understood. Upon such remains 

 have been founded several genera (Eocystites, Trochocystites, Proto- 

 cystites, &c), but these do not at present admit of precise definition. 



1 In such Urchins as Scutella (fig. 240) the branches of the ambulacral grooves 

 on the actinal surface extend into the interambulacral regions, and are lined by 

 minute tube-feet, while in Echinanthns there are isolated "locomotive" pores 

 scattered over many of the interambulacral plates. 



