BLASTOIDEA. 



461 



of each ambulacral field there was situated, however, a single or 

 double row of slender jointed " pinnules." The sockets for these 

 are placed between each pair of marginal pores, but the delicate 

 pinnules themselves are rarely preserved. 



In perfect specimens of the Blastoids, the apex of the ventral sur- 

 face (fig. 328, c) appears to have been closed by a canopy of small 

 calcareous plates, some of which represent the orals of Crinoids ; 

 but in the majority of examples these have been destroyed in the 

 process of fossilisation. In the Blastoids, therefore, as in the 

 Palaeocrinoids generally, the mouth was really subtegminal, and was 

 not visible externally in the natural condition. When the covering- 

 plates, however, have been lost, the mouth is seen as a central aper- 

 ture placed at the point of convergence of the ambulacral fields 

 (fig. 328, d). The vault of plates concealing the mouth passes 

 laterally without a break into the " covering-plates " of the ambu- 

 lacral grooves. The anus is excentric, and pierces one of the 

 interradials ("deltoid" plates). 



The most remarkable organs in the Blastoids are the respiratory 

 tubes or " hydrospires," which are probably always present in one 

 form or another, though they have not 

 been universally detected. In Pentre?nites 

 (fig. 330) the hydrospires lie underneath 

 the lateral portions of the ambulacral 

 fields, one on each side of each ambu- 

 lacrum, so that there are ten of these 

 organs in all. When the lancet-plate is 

 removed from an ambulacrum in this 

 genus, the outer sides of the hydrospires 

 are seen on each side (fig. 329, c, /z), 

 with the "under lancet - plate " in the 

 centre. Each hydrospire consists of a 

 flattened lamellar tube, with thin calcare- 

 ous walls, more or less extensively folded, 

 and terminating internally by closed and 

 somewhat dilated ends, the reduplications 

 of the tube being placed lengthways be- 

 neath the ambulacrum. Water is admitted (After zittei.) 

 into the hydrospires by slits on their outer 



faces in direct connection with the rows of " marginal pores " between 

 the overlying "side-plates" of the ambulacrum (fig. 330,//). At 

 their upper ends the hydrospires are placed in further communica- 

 tion with the exterior by a ring of five or ten openings surrounding 

 the peristome. These apertures (formerly called " ovarian open- 

 ings ") are usually spoken of as the "spiracles" (fig. 328, c), and 

 they probably served for the escape of water from the hydrospires. 



Fig. 330. —Cross-section of the 

 calyx of Pentremites sulcat7is, 

 from the Carboniferous rocks of 

 Illinois, somewhat enlarged, show- 

 ing the transversely divided "hy- 

 drospires " (/zy) ; rr, Radial plates; 

 /, Lancet-plate ; p p, Side-plates. 



