THE BLASTO1DEA 



Asteroblastus and Blastoidocrinus ; the position of the anus, in AsteroUastus ; 

 the hydrospires, though far less developed, in Blastoidocrinus. So much 

 is this the case that Cadaster has been referred to the Cystidea by several 

 writers of eminence. Further arguments for such action are found in the 

 fact that Cadaster has no " spiracles " at the proximal ends of the ambu- 

 lacra, and no hydrospire-pores along their sides, 

 structures which are often considered character- 

 istic of the Eublastoidea. The gradual evolution 

 of these structures may, however, be traced 

 within the order. 



Phaenoschisma, Etheridge & Carpenter (1882, 

 -86), Devonian and Carboniferous, Europe and N. 

 America, differs from Cadaster mainly in the fact 

 that the hydrospire-folds become more concen- 

 trated, and pushed in under, or overgrown by, 

 the side-plates and part of the lancet-plate, so 

 that as a rule only their ends are visible (Fig. 

 VII. 2) ; they are also well developed in the 

 anal IR. The side-plates may lie at the sides 

 of the lancet-plate, as in Cadaster, or they may 

 lie on it ; in cases where the proximal side-plates 

 project far over the hydrospires, they may 

 entirely roof in the depression in which the 

 hydrospires lie, only leaving a small communi- 

 cation with the exterior on either side the 

 proximal end of the ambulacrum (Fig. VII. 6). 

 The openings thus formed are the rudiments 

 of " spiracles." The side-plates that in Cadaster 

 formed the outer halves of the rhombohedral 

 pairs are here diminished in size and pushed out- 

 wards (Fig. VII. 1). They are now distinguished 



as " outer side-plates " (cf. similar structures in the recumbent arms of 

 Callocystinae). Cryptoschisma, Etheridge & Carpenter (1886), Lower 

 Devonian, Spain, differs from Phaenoschisma and Cadaster in little but the 

 greater breadth of the ambulacra, which entirely conceal the eight hydro- 

 spire-slits on either side. Sometimes the ambulacra do not reach to the 

 tops of the deltoid crests (or oral ridges), and thus there is a spiracle on 

 either side of each crest, or ten in all ; but sometimes they reach right up 

 the oral ridges, so that the spiracles of adjacent ambulacra become con- 

 fluent, and there are five in all (Fig. VII. 8). This is expressed by saying 

 spiracles " double " or " single " respectively. Orophocrinus, von Seebach 

 (1864 ; synn. Dimorphicrimis, d'Orb. ; Codonites, Meek & Worth), Carboni- 

 ferous, Britain, Belgium, and N. America (Fig. IV.). The hydrospire-slits 

 are more concentrated at the bottom of the depression for the ambulacra 

 (" radial sinus ") than in previous genera ; but since the ambulacra are 

 narrower, some of the slits may be exposed, and the spiracles are merely 

 clefts, often extending all along the sides of the ambulacra (Fig. VII. 7). 

 The concentration of the hydrospire-slits causes the inner walls of the 

 two nearest the median line of the ambulacrum to meet along that line, 



FIG. VI. 



Food -groove and associated 

 structures of Cadaster trilaba- 

 tus, greatly enlarged, o.s.p, 

 outer side -plate; s.p, side- 

 plate ; Ir, brachiole - facet, 

 from which passes a small 

 groove tof.g, the main food- 

 groove, along which are im- 

 pressions of covering-plates. 

 These structures are borne 

 by L, lancet-plate, in which 

 is n.c, a possible nerve-canal. 

 Beneath this emerges h.p, 

 the plate forming the side 

 of h, the first hydrospire-fold. 

 (Based on Brit. Mus. E8027.) 



