BLASTOIDEA 



599 



The diplopores of the Diploporita appear to consist of two 

 canals traversing the body-wall, opening close together into a 

 common pit externally, but diverging internally. Since in some 

 cases, as in Aristocystis (Fig. 278), this common pit is' proved to 

 have been closed externally by a very 

 delicate layer of calcification, it is 

 probable that the pores represent 

 in other cases the points of origin 

 of finger-like gills similar to those 

 of Asteroidea. Where they were 

 closed by calcification this was so 

 thin and porous that the diffusion 

 through it sufficed for respiration. 

 Jaekel regards the Diploporita as a 

 group derived from Dichoporita, but 

 this seems to be extremely doubtful. 



CLASS V. BLASTOIDEA ^,,.^,s.-AHstocysUs. mtheupper 



. part of the calyx the heavy dots 



PelmatOZOa with respiratory are "diplopores," seen owing to 



organs in the form of longitudinal ("zittehj" '""^''^"'^^ ^'■^''■ 

 calcified folds, termed " hydrospires," 



radiating from the mouth. Stem well developed ; calyx regular, 

 consisting of a whorl of basals surmounted by a whorl of forked 

 radials, in the clefts of which lay the recumbent radial water- 

 vascular vessels, supported each on a special plate (" lancet 

 plate "), and giving off two rows of branches supported by short 

 fingers (Fig. 279). Side-plates and covering plates were also 

 developed ; five orals (" deltoids ") completed the calyx. The 

 anus was at the side, just beneath one of the orals. 



The hydrospires, which are the great characteristic of the 

 class, are seen in section in Fig. 279, B (hyd). They consist of a 

 varying number of parallel folds on each side of each " pseud- 

 ambulacrum," as the lancet plate with its adhering side-plates 

 and covering plates has been termed. In the most primitive 

 genus. Cadaster, they appear to have opened directly to the 

 exterior, and to have been placed at right angles to the lines of 

 union of the radial and oral plates, just like the grooves of a 

 pectinated rhomb. In more modified forms, such asFentremites 

 and Granatocrinus (Fig- 279), the outer openings were overarched 



