462 



PELMATOZOA. 



The general arrangement of the hydrospires in the majority of the 

 Blastoids is essentially similar to that which obtains in Pentremites, as 

 above briefly described ; but there are some remarkable departures from 

 this type. Thus in Codaster ( = Codonaster) there are only eight hydro- 

 spires, and these open externally by slits separated by intervening ridges, 

 which occupy four of the five interradial areas on the ventral surface of 

 the body, and which have a direction parallel with that of the ambulacra 

 (fig. 331, a). In Phcenoschisma the arrangement is the same, but the 



Fig. 331.— a, Upper surface of the calyx of Codaster trilobatus, var. acutus, from the Car- 

 boniferous Limestone, enlarged, showing the numerous hydrospire-slits (/) in all the interradii 

 except the anal one. (After M'Coy.) b, Side-view of the calyx of Oropkocrinus (Codonites) 

 stelliformis, from the Carboniferous Limestone of North America, of the natural size, c, Upper 

 surface of the calyx of the same (after Meek and Worthen), enlarged, a, Anal aperture ; r, 

 Ambulacra; p, in fig. c, Hydrospire-clefts or "spiracles." 



hydrospire-slits occupy the anal as well as the other four interradii. 

 Lastly, in the genus Orophocrmus (fig. 331, c) the " spiracles " or external 

 openings of the hydrospires are in the form of ten clefts, of variable 

 length, which extend along the sides of the ambulacra. 



There can be little doubt that the " hydrospires " of the Blastoids, 

 like the less highly developed structures of the same kind in the 

 Cystoids, are essentially respiratory in function. They present in 

 many respects a remarkable resemblance to the folded genital 

 "burs3e"of the Ophiuroids, as regards both their structure and 

 their mode of termination on the surface of the body. It is 

 therefore probable that the hydrospires, like the " bursae " of the 

 Brittle-stars, were connected with the discharge of the reproductive 

 elements into the external medium. 



As regards the distribution in time of the Blastoids, no member 

 of the order has been as yet certainly detected in the Ordovician 

 rocks. In the Silurian rocks of North America a few forms have 

 been found, and in the Devonian deposits both of the New and 

 Old World various genera are represented, such as Pentremitidea, 

 Eleutherocrinus, Codaster, and Elceacrinus. It is in the Carbonifer- 

 ous rocks, however, that the Blastoids attain their maximum devel- 

 opment, the type-genus Pentremites (fig. 332, a and b) — sometimes 

 written Pentatremites or Pentatrematites — appearing here for the 



