CYSTIDEA. 



307 



middle layer traversed by canals, the mesostereom ; and a thin smooth 

 inner layer, the hypostereom. In many Cystidea the middle layer is 

 traversed at right angles to the surface by canals, the ends of which are 

 closed by the epistereom and hypostereom, if these layers are preserved. 

 If the latter are weathered off the canals appear to open in surface pores. 



BcL 



Flo. 215. Asteroblastus Volborthi (after 

 Schmidt and Bather, from Delage and 

 Herouard). dlt oral plates ; p pinnules; pd 

 stalk ; R radial plates. 



FIG. 216. Echinosphaera aurantium (after 

 Volborth). an anus ; b mouth ; mdp 

 water-pore. The stalk projects below. 



These pores may occur singly (haplopores) or in pairs (diplopores). When 

 the canals are in pairs, the external openings of a pair are placed in a 

 common pit on the stereom. *| 



In some forms, classed as Rhombifera, canals are found in the meso- 

 stereom traversing the plates parallel to the surface ; these canals extend 

 across the suture, at right angles to it, to be continuous with similarly 

 arranged canals on the neighbouring plates (Fig. 218). They are so ar- 

 ranged that their terminations form 

 a rhombic figure, the diagonal of 

 which is occupied by the suture 

 between the plates concerned. For 

 this reason and because the ter- 

 minations of the canals often bend 

 outwards and appear on the surface 

 as pores, the figures caused by them 

 are called pore-rhombs (Fig. 218). 

 At their ends, and sometimes also 

 near the suture, the canals may 

 also bend or send a branch inwards 

 to the inner surface of the meso- 

 stereom. Prof. Jaekel compares 

 these terminal vertical canals with 

 the two canals of a diplopore, and 

 supposes the horizontal canal to 

 represent the peri-poral depression greatly extended. 



In some forms, however, the canals of the rhombs have the form of 

 grooves, the sides of which project on the external surface of the plates, 

 so that the stereom appears as though thrown into folds. Pectini -rhombs 

 are pore-rhombs in which such folds or grooves are very deep and at the 



FIG. 217. Protocrinus oviformis (after Vol- 

 borth from Delage and Herouard). an 

 anus ; b mouth ; mdp water-pore. 



