ECHINODERMATA - KU ScholarWorks - University of Kansas

BLASTOID STUDIES 107
schismatidae ETHERIDGE St CARPENTER, 1886). The most
primitive form is Polydeltoideus, with ten exposed
fields, six anal deltoids, steeply conical form, and
lancet covered by side plates. One trend is toward reduction
of the number of hydrospire folds on the
anal side (Deltoschisma), from which group presumably
came blastoids that developed an adoral epideltoid
plate or in which the lancet plate migrated
outward (Phaenoblastus). Another trend is one in
which the cryptodeltoids fuse to form a subdeltoid
plate (Decaschisma), and the ambulacra move out
laterally. A plausibly distinguished trend seems to be
one in which the ten fields close and pores and spiracles
(five) are formed (Hyperoblastus), but the aboral
ends of the exposed slits remain. A fourth trend may
be one in which the ambulacra move downward, then
laterally outward, concurrent with outward migration
of the hydrospire fields (Pleuroschisma), with
reduction or fusion of the superdeltoid and cryptodeltoids
to form an epideltoid (Conoschisma). This
last mentioned genus probably gave rise to Nymphaeoblastus,
in which the ambulacra continued to move
downward, to Neoschisma and Notoblastus, in which
the ambulacra moved outward from the oral area and
atrophied on the summit, to Dipteroblastus, in which
three ambulacra completely atrophied on the summit,
and to T haumatoblastus, in which the ambulacra extended
outward into long wings. Thus, the abovementioned
genera seem to form a natural family.
Another natural family (Orophocrinidae JAEKEL,
1918) is considered to be one characterized by the
presence of ten spiracular slits. The most primitive
known blastoid of this group is Orophocrinus, which
may have been derived from Pleuroschisma by fusion
of the cryptodeltoids and superdeltoid into an epideltoid
and closure of the sinuses to form ten spiracular
slits. Orophocrinus probably gave rise to Anthoblastus
by outward migration of the lancet plate, to Sphaeroschisma
by downward migration of the ambulacra
and atrophy or reduction in size of the hypodeltoid,
and to Pentablastus by fusion of the epideltoid and
hypodeltoid to form an anal deltoid, retaining the
elongate spiracular slits in all genera. A trend in this
group is reduction in length of the spiracular slits to
form small round slits on the summit and shape of
the calyx becoming spherical (Sundablastus), or in
addition the ambulacra migrate away from the oral
opening (lndoblastus) and the ambulacra become
short, with winglike interradial projections (Timoroblastus),
or the ambulacra migrate away from the oral
opening, become short, and the hypodeltoid becomes
small or atrophied (Microblastus). These trends seem
to be logical and therefore the mentioned genera may
belong to a natural family.
A fourth family of fissiculate blastoids (here
designated as Brachyschismatidae, FAY, n. fam.) is
conceived to be one in which the hydrospire fields
have been reduced to nine. This assemblage includes
the single genus Brachyschisma, which was probably
derived from Decaschisma by reduction of the hydrospire
field on the left (D side) of the anal opening,
so that only nine fields remained.
The fifth and remaining family (Codasteridae
ETHERIDGE & CARPENTER, 1886) is one with eight exposed
hydrospire fields. The primitive forms of this
group is Trionoblastus, which has eight hydrospire
fields exposed and three anal deltoids. Trionoblastus
was probably derived directly from Decaschisma by
reduction of the hydrospire fields in the anal region,
thus leaving eight exposed hydrospire fields. It is also
possible that the hypodeltoid became small or atrophied,
since it is generally absent in this family. Trionoblastus
probably gave rise to Codaster as result of
fusion of the subdeltoid and superdeltoid to form an
epideltoid plate. Codaster probably gave rise to Paracodaster
by reduction or atrophy of the slits on the
radial plates, to Angioblastus and Sagittoblastus by restriction
of the hydrospire fields to deep slits, and to
Pterotoblastus by formation of radial wings.
Thus, the order Fissiculata may include five natural
families differentiated on the basis of having (1) ten
exposed hydrospire fields, (2) nine exposed hydrospire
fields, (3) eight exposed hydrospire fields, (4)
ten spiracular slits, and (5) eight spiracular slits. These
respectively are appropriately named Phaenoschismatidae
ETHERIDGE & CARPENTER, 1886 (nom. correct. pro
Phaenoschismidae); Brachyschismatidae FAY, n. fam.;
Codasteridae ETHERIDGE & CARPENTER, 1886; Orophocrinidae
JAEKEL, 1918; and Astrocrinidae AUSTIN &
AUSTIN, 1843.
PHYLOGENETIC TRENDS OF
SPIRACULATA
Two diverse origins of the five families here recognized
in the order Spiraculata may be considered. One
family of blastoids (Pentremitidae) with five oval
spiracles seems to have been derived from Hyperoblastus,
a Devonian genus. The other four families
severally characterized by having five paired spiracles;
nine spiracles; ten spiracles with anus separate; and
ten spiracles, anus separate, and two cryptodeltoids