ECHINODERMATA - KU ScholarWorks - University of Kansas

BLASTOID STUDIES 15
ture that may have developed independently in several
diverse stocks of blastoids.
FUSED HYDROSPIRE PLATE
A skeletal element of blastoids termed sublancet
plate by previous authors is identical to the hydrospire
plate in all respects except two. In the radial portion
of the plate, extending admedially from the aboral
tip of an ambulacrum almost to the position of the
radiodeltoid suture, the admedial walls of adjacent
admedial hydrospire folds fuse to form a thick plate
beneath the lancet plate. In addition, the sublancet
plate may occur in blastoids lacking pores (Orophocrin
Us, Pentablastus). The only pore-bearing blastoid
in which it is described is Cal ycoblastus.
Because this structure is produced by fusion of two
admedial hydrospire plates along an ambulacrum, the
term sublancet should be suppressed in favor of fused
hydros pire plate, thus showing the derivation of the
structure from hydrospires. The so-called sublancet is
not connected with the lancet plate in origin, shape, or
function, and hence another name should be given to
this element.
The fused hydrospire plate is here interpreted to
be an advanced, specialized respiratory structure resulting
from adorai migration, outward migration,
and thickening of admedial walls of the admedial
hydrospire folds beneath the lancet plate. This structure
is absent in pre-Mississippian blastoids.
HYDROSPIRE PORES
Hydros pire pores are openings that communicate
between the hydrospire canals and exterior. Generally
the pores are excavated in the adambulacral margin
of a deltoid or radial and are shared partially by the
adlateral corner of the aboral edge of a primary side
plate where they meet the adorai edge of an adjacent
outer side plate. If a hydrospire plate is present, the
pores are located between this plate and the adjacent
radial or deltoid.
In specimens of Mesoblastus there are approximately
five pores for each side plate, with an admedial
set of three and an admedial set of two seemingly arranged
in two rows. This feature seems to denote
nothing more than infolds in the hydrospire plate and
the adjacent adambulacral margin of the radial whereby
a fold near the interior becomes folded once or
twice externally, and no evidence is found of two
separate sets of pores with different functions.
In the genus Hyperoblastus pores seemingly are
formed as gaps between side plates, but in this group
the aboral ends of the hydrospire slits are exposed.
This affords excellent argument in favor of derivation
of the Spiraculata from the Fissiculata by means of
infolding of hydrospire fields and consequent formation
of pores as gaps between side plates. With continuing
evolution it is quite possible that the pores
migrated laterally into the substance of the radial and
deltoid plates and that these plates subsequently began
to infold along their margins to form many
pores along each side plate. This probably took place
independently in Europe and North America in unrelated
stocks.
ORAL PLATES
An oral plate is one of the five plates covering part
of the oral area, interradial in position, generally pentagonal
in outline, aborally adjoining a deltoid and
adoral parts of two adjacent ambulacra, and meeting
other orals at a common point above the oral opening.
The anal oral, or oral plate of the anal interradius,
is larger than the other four and generally sixsided.
These plates are best seen in Nucleocrinus.
In some genera many accessory plates surrounding
the orals and extending above the side plates for a
short distance are seen. These accessory orals meet the
cover plates of the ambulacra so as to conceal the entire
area around the mouth and adjacent food grooves.
Such plates are best seen in Placoblastus.
In Pentremites an additional structure, termed oral
pyramid, is seen to be composed of specialized elongate
brachiolar plates, which overlap elongate oral
plates covering the mouth. In addition to the brachiolar
plates, specialized side plates appear to cover most,
if not all, of the spiracles. The spiracles probably had
outlets toward the inside aboral part of the pyramid
and above each ambulacral groove. The only external
outlets seem to be large openings above the food
grooves, located at the point where the food grooves
enter the oral pyramid.
The important conclusion from these observations
is that the deltoids are not at all interpretable as oral
plates, that, is homologous with the oral plates of
crinoids.
ANAL PLATES
The small polygonal plates covering the immediate
anal opening are termed anal plates, as shown in
Orophocrinus stelliformis. In Pentremites these appear
to be imbricate triangular plates. In most bias-