CLASS II 



BLASTOIDEA 



165 



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

 above noted, 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 th^ 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 confluent, PentremitcssuieatusSay. Lower 

 the canals of adjacent groups of hydrospires enter the ^e"^°Sr Lf 'clS\t ''.bout 

 same opening. The hydrospires are suspended in the Vs pe height of the ambulacral 



. -^ j.% 1 ,1 n j-.ii 1 -^ fields, xli/2- /iy, Hydrospires; 



majority oi lorms along the walls oi the body-cavity i, Lancet-piate ; p, Pore-piates; 

 (Pentremites, Fig. 257), being attached either to the '"- ^•■^^iais. 

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

 hydrospire plate (Orhitreniites, Mesohlastvs and Cryptoblastus). Pentremites has 



Fig. 257. 



A 



'^^^TKc/ 



Fig. 25S. 



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

 derhi/ennif:. B, Orbit remites nonuoodi. C, Metablastus lineatus. DjOrophocrlnus verus. All sections enlarged 

 (after Etheridge and Carpenter). 



from four to nine hydrospires in each group ; Orbitremites two, or exceptionally 

 one ; Troostocrimis and Mesohlastiis generally three, and Orophocrinus from five 

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



