CLASS HI 



BLASTOIDEA 193 



ncath the lancet- and side-plates, in a direction parallel with the boundaries of 

 tin- anilMilaeral tit-Ms. They begin at the lower end of the ambulacra, and 

 terminate in the hydrospire canals, of which the spiracles form the external 

 apertures. When the spiracles are confluent, the canals of adjacent groups of 

 hvdrtispin-s i nter t he sanu- opening. The hydrospires are suspended in the 

 majority of forms along the walls of the body-cavity (Pentremites), (Fig. 317), 

 l.cinu r attached either to the outer margins of the under lancet-plate or to a 

 separate piece known as t he ln/<lrospire plate (Granatocrinus, Mesoblastus, and ('////>/ 

 M/X///S). l'< nfirmitt's has from four to nine hydrospires in each group ; Granato- 

 ri'imix two, or exceptionally one; Troostocrinus and Mesollastus generally three, 

 and <)ri<1i,Tinit* from five to seven (Fig. 318, A to D). In Phaenoschisma and 

 I'lH/imfi-r (Fig. 323) the tubes open externally by slits piercing the radials and 

 deltoids and running parallel with the ambulacra. 



The functions of the hydrospires can only be surmised, but they are 

 supposed to have served for respiration ; they correspond doubtless to the 

 pet 'tinated rhombs and calycine pores of the Cystideans and to the respiratory 

 pores of Crinoids. It is probable that water was admitted to the hydrospire 

 sacs through the marginal pores, and was discharged through the spiracles. 

 Roemer and Forbes have suggested that the hydrospires may also have per- 

 formed reproductive functions. Ludwig has called attention to the resemblance 

 between the genital bursae of Ophiuroids and the slit-like spiracles in Oropho- 

 rriiiiix: his theory is that the hydrospires served both for purposes of respiration 

 and for the discharge of genital products, a view which was also shared by 

 Carpenter. 



The stem in Blastoids is preserved only in exceedingly rare instances. It 

 is round, provided with a small axial canal, and composed of short joints, 

 which apparently multiplied in a similar manner as in the Crinoids. In Oro- 

 phocrinus and Pentremites it has been traced for a length of 15 cm. without 

 reaching the end ; and in the latter form it has occasionally been found with 

 a few, comparatively heavy, radicular cirri. 



It has frequently been claimed, owing to the superficial resemblance of their 

 ambulacral areas, that the Blastoids and Echinoids are mutually related ; but 

 such presumptions are founded upon a total misconception of the value of 

 external characters. The construction of the calyx, the presence of pinnules, 

 and the stemmed condition, are features which identify them unmistakably as 

 Pelmatozoa ; and their nearest relatives under this group are the Cystideans. 

 The parallelism between the ambulacral fields of the one class and the recumbent 

 arms, apparently soldered on to the calyx of the other, is self-evident. The 

 hydrospires of Blastoids correspond to the pore-rhombs of Cystideans, as has 

 already been remarked ; and the position of the mouth and anus is the same 

 in both types. The Blastoids constitute a peculiar, but, on the whole, a very 

 well-defined group, which is now regarded as of equal rank with the Crinoids 

 and Cystids. 



Blastoids have not been recognised as such, up to the present time, in 

 strata lower than the Silurian ; but it is possible that several genera occurring 

 in the Ordovician of North America and Russia (Blastoidocrinus, AsteroUastus, 

 etc.), which are now referred to the Cystids, may eventually be transferred to 

 the Blastoidea. The only known Silurian form is Troostocrinus, which occurs 

 sparsely in the Niagara Group of North America. Several genera are repre- 

 sented in the Devonian, being distributed in both Europe and America, but 

 VOL. I 



