ECHINODERMATA - KU ScholarWorks - University of Kansas

10 THE UNIVERSITY OF KANSAS PALEONTOLOGICAL CONTRIBUTIONS
quadrisecting each large basal. In the small azygous
basal, a ridge extends from the middle of each of the
two distal edges bordering the radials to a common
point at its proximal edge, thus trisecting this plate.
The interbasal sutures comprising the junctions of
each pair of juxtaposed basal plates are located in
shallow depressed areas between the grouped raised
ridges of the basals. The angle between interbasal
sutures, as measured from their adoral extremities to
the center of the stem is 140 degrees for those bounding
either of the two large basals and 80 degrees for
those bounding the azygous basal. However, near
the center of the stem, the two lateral sutures of the
azygous basal curve rapidly toward each other so
that near their intersection at the stem center, the
angle subtended by them is 120 degrees.
Growth of the basais proceeds by uniform accretions
outward admarginally from the subcenter of
each plate and accordingly growth lines, where perceptible,
are parallel to the margins of the plate, with
relatively wide spaces between lines parallel to the
basiradial suture, narrower spaces between those parallel
to the interbasal suture, and almost imperceptible
spaces between those adjacent to the stem.
In forms having only a single basal plate, this consolidated
plate may be formed in two ways. The discrete
plates of the basal circlet may have fused into
one large plate (as seems true in Acentrotremites), or
a small plate remained in the position of the "AB"
azygous basal while the other two were resorbed
(Astrocrinus). In Zygocrinus, the small plate is eccentric,
being located on the side of the base of the calyx
and is in the normal position of the "AB" basal.
The basals are seemingly not diagnostic for generic
determinations for apparently they functioned
merely as support for the calyx. Nothing is known at
present about the nervous system and until something
of this is learned, no comparison with other pelmatozoa
ns can be made. In no known blastoid do the
basals receive ambulacra nor do they contain pores or
slits of any type. In this respect the blastoids differ
from various genera of rhombiferoid cystoids. The
basal plates were a remarkably conservative and stable
element of the calyx throughout their history.
RADIALS
Situated above the basals are five radial plates, each
usually cleft more or less deeply by a V-shaped sinus,
which receives an ambulacrum. The body of the radial
is the main portion of the plate located aborally with
respect to the tip of the sinus. The limbs are portions
of the radial that border the sides of the ambulacrum.
The radial lip is the thickened tip of the radial sinus.
If ridges occur on the outer surface, the "C" (right
posterior) and "E" (left anterior) radials, have a
single ridge extending longitudinally along the middle
from the radial lip to the mid-point of the basiradial
suture, whereas the other three radials have two ridges
extending from the radial lip to the lower corners.
These are characters correlated with the pattern of
the basal circlet, together defining an axis of symmetry
that passes through the "D" (left posterior) radius and
"AB" (right anterior) interradius, although this
neglects location of the anal opening; presumably the
symmetry indicated is a rather fundamental character
which may bear on the question of origin of the
blastoids.
Growth of the radials proceeds by accretions marginal
to the radial lip, and thus growth lines are concentric
about this point, with wider spaces between
lines parallel to the basiradial suture and narrower
spaces between those parallel to the interradial sutures.
At the aboral tip of the ambulacrum, the radial
plate may extend beneath the lancet plate and the
hydrospires may terminate well within the substance
of the radial. The radial plates normally bear the
aboral portions of the hydrospires.
The radial plates seem to have two functions: (1)
to receive the aboral extremities of the ambulacra, and
(2) to aid in respiration by forming infolds along the
sides of the ambulacra. The nature of the overlap of
the radials with the deltoids is probably of generic
importance and is a constant character in any one species.
DELTOIDS AND SPIRACLES
The most important morphological elements in
blastoids, as indicated by their significance in taxonomy
and phylogeny, are the deltoid plates and their
associated structures, for on the nature of these parts
most generic distinctions are primarily based. The
deltoid is so named because of its generally subtriangular
outline when viewed in a direction normal to
its outer surface; this shape is explained by the manner
in which it is wedged between adjacent ambulacra
and terminated aborally against, above, or below the
distal extremities of the radial limbs. A single deltoid
occurs in each interradius (generally more than one
on the anal side), with sutures termed radiodeltoid
sutures, along its aboral margin adjoining the radial
limbs.