CLASS III BLASTOIDEA 193 
neath 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 external 
apertures. When the spiracles are confluent, the canals of adjacent groups of 
hydrospires enter the same opening. The hydrospires are suspended in the 
majority of forms along the walls of the body-cavity (Pentremites), (Fig. 317), 
being attached either to the outer margins of the under lancet-plate or to a 
separate piece known as the hydrospire plate (Granatocrinus, Mesoblastus, and Crypto- 
blastus). Pentremites has from four to nine hydrospires in each group ; Granato- 
crinus two, or exceptionally one; T'roostocrinus and Mesoblastus generally three, 
and Orophocrinus from five to seven (Fig. 318, 4 to D). In Phaenoschisma and 
Codaster (Fig. 323) the tubes open externally by slits piercing the radials and 
deltoids and running parallel with the ambulaera. 
The functions of the hydrospires can only be surmised, but they are 
supposed to have served for respiration; they correspond doubtless to the 
pectinated 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- 
crinus, 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, Asteroblastus, 
etc.), which are now referred to the Cystids, may eventually be transferred to 
the Blastoidea. The only known Silurian form is Troostocrinus, which oceurs 
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 O 
