ECHINODERMATA - KU ScholarWorks - University of Kansas

14 THE UNIVERSITY OF KANSAS PALEONTOLOGICAL CONTRIBUTIONS
deltoids, thus showing migration of the lancet plate
in an aboral direction.
HYDROSPIRES
An infold of calcareous tissue (stereome) developed
from a radial and adjoining deltoid plate, generally
at right angles to the radiodeltoid suture and
subparallel to the adjacent ambulacral margin, is
termed a hydrospire fold or hydros pire. Multiple hydrospire
folds along one side of an ambulacrum comprise
a hydrospire field or hydrospire group. The hydrospire
slit or cleft is a fissure-like opening of a
hydrospire fold. If there is no direct communication
to the outside and the slits open into a common canal,
the canal is termed a hydrospire canal. On the anal
side of some blastoids, the epideltoid, subdeltoid, or
cryptodeltoids may be infolded into hydrospires, or
hydrospires may be absent on this side, or present on
one side and not on the other, or reduced in number.
Accordingly some genera and species are distinguished
by these criteria.
Aboral extremities of the hydrospires end in substance
of the radial limbs and never extend aborally
beyond the aboral tips of the ambulacra. Adorai extremities
of the hydrospires of blastoids belonging to
the order Spiraculata terminate between small septa
attached adaxially to each of two septa that extend
laterally from the deltoid septum and meet the underside
of the lancet plate. The longest hydrospire fold
is admedial to an ambulacrum and the shortest fold
is adlateral to an ambulacrum or deltoid. Invariably
the wall of the longest fold adjacent to the lancet
plate is perfectly parallel to and in constant contact
with the lancet plate.
Two schools of thought concerning the function
of hydrospires may be recognized. These respectively
hold that (1) hydrospires are homologous with bursae,
and alternatively, (2) hydrospires are homologous
with skin gills. Bursae are small internal interradially
disposed saclike organs found in modern echinoderms;
they function in reproduction and communicate
with the exterior by means of a large orifice. Although
two bursae generally occur in each interradius,
with openings adjoining each other, more than two
in each interradius may occur. Skin gills are thin areas
of the calyx in some living echinoderms, developed
where the ectoderm and mesoderm meet so as to allow
respiration to take place. In modern forms these
areas have no constant position, but in some fossils
(e.g., cystoids) structures (pore-rhombs) comparable
to skin gills have a regular arrangement with respect
to sutures. A pore-rhomb is a diamond-shaped area of
stereome infolds, the longest extending between centers
of two adjacent plates and at right angles to the
suture between the plates. Successively shorter folds
occur on either side of the main fold and are parallel
to it, the shortest folds occurring at the corners of the
rhomb farthest removed along the suture from the
main fold. In order to derive a hydrospire from a porerhomb
it is necessary that half of the pore-rhomb
should become atrophied so as to leave one-fourth of
its area on a deltoid and another fourth on an adjacent
radial limb. BILLINGS (1869), JAEKEL (1918),
and MOORE (1954) have considered that it is rational
to derive hydrospires in this manner, even though no
evidence of such atrophy has ever been demonstrated
in the cystoids. CLINE (1944) has suggested that
hydrospires represent successive infolds along margins
of the ambulacra, which seems rational and in
line with ontogenetic growth.
The weight of evidence tends to show that hydrospires
are a distinct structure peculiar to blastoids and
lacking in other known echinoderms, and therefore,
the blastoids should be segregated as a class. Although
bursae are comparable to internal hydrospires they do
not correspond to exposed hydrospires (Fissiculata).
Pore-rhombs must atrophy in position and manner
not known in cystoids, if they are recognized as antecedent
structures. There is a distinct possibility that
bursa-like organs may have been attached to the abaxial
walls of the hydrospires, thus emptying eggs and
sperms into a respiratory structure. There is also the
possibility that adaxial extremities of the folds may
have functioned for reproduction while other portions
may have served for respiration, or the hydrospires
may have had nothing to do with reproduction.
HYDROSPIRE PLATE
The hydrospire plate, present only in pore-bearing
blastoids, is structurally the infolded wall of the
adambulacral margin of a radial or deltoid plate adjacent
to the lancet plate, folded back against the
adjacent adambulacral margin of a radial or deltoid
plate. Its presence is detected by the appearance of
pores beneath the side plates or lancet plate, where
these are removed, or by pores occurring entirely within
a radial plate or deltoid plate, or the absence of
hydrospire slits where the lancet is missing, or the
presence of many more pores than side plates. In some
genera the hydrospire plate is concealed by the radial
and deltoid margins. This plate is a specialized struc-