l.,, 



BLASTOIDEA 



165 



The crenulations, or file-like markings across the ambulacra in Pentremites 

 above iioted, are not mere surface ornamentation ; but the ridges constitute 

 the sides, and the depressions the floor, of a series of small ducts leading from 

 the pinnules, and forming lateral branches of the main ambulacral groove 

 into which they discharge. These lateral ducts, as well as the main median 

 groove of the ambulacrum, are, in well-preserved specimens, roofed over 

 throughout the entire field by very minute alternating covering plates extend- 

 ing all the way to the pinnules, and probably continuing along their ventral 

 side. In this respect the structure of the ambulacral area has not been 

 generally understood, and not heretofore correctly described. The arrange- 

 ment of the side ducts, their discharge by a distinct curvature into the main 

 groove, and their connection with the pinnules, leave no doubt that they 

 were the closed food-grooves serving to conduct nutriment from the pinnules 

 on toward the mouth. 



In most Blastoids the side-plates, or the outer side-plates when such are 

 present, are pierced by marginal pores (or hydrospire pores), which communicate 

 with the hydrospires. The pores are situated at the extreme outer margins 

 of the plates, at the end of the lateral ridges, and 

 alternate in position with the sockets of the pinnules. 

 They are present in all forms having the hydrospires 

 concealed within the calyx ; but are absent in the 

 Codasteridae, in which the hydrospires are wholly or 

 in part exposed on the outer surface. 



The hydrospires (Figs. 257, 258) are bundles of 

 flattened, lamellar tubes, extending underneath the 

 lancet- and side-plates, in a direction parallel with 

 the boundaries of the ambulacral fields. They begin 

 at the lower end of the ambulacra, and terminate in 

 the hydrospire canals, of which the spiracles form the 

 adoral apertures. When the spiracles are confiuent, 

 the canals of adjacent groups of hydrospires enter the J; 



same opening. The hydrospires are suspended in the Vs^^the hdght «f^tue^^^^bi^^^^^^^ 

 majority of forms along the walls of the body-cavity i, LaAcet-piate ; p, Pore-piates; 

 (Pentremites, Fig. 257), being attached either to the ^' 



outer margins of the under lancet-plate or to a separate piece known as the 

 hydrospire plate (Orbitremites, MesoUastus and Cryptohlastus). Pentremites has 



--^^..f 



Pentremites sulcatusSaj. Lower 

 Carboniferous ; Illinois. Trans- 

 erse section of calyx at about 



mps)^ 



Fig. 258. 

 Transverse sections through the ambulacral fields, showing various forms of hydrospires. A, Orhitremites 

 derhijemis. B, Orbitremites nonoooiU. C, Metablastus lineatus. D, OropTiocrinus verus. All sections enlarged 

 (after Etheridge and Carpenter). 



from f our to nine hydrospires in each group ; Orhitremites two, or exceptionally 

 one ; Troostocrinus and MesoUastus generally three, and Orophocrinus from five 

 to seven (Fig. 258, A-D). In Phaenoschisma and Codaster (Fig. 260) the tubes 



