Phylogenetic relationship undetermined due to
significant contradictions of evidence or lack of
Placozoans are morphologically similar to poriferans in many ways, though others argue that they may be degenerative cnidarians (Pechnik 2000; p. 86).
This grouping (clade) of platyhelminthes, rotifers, and lophotrochozaa is based primarily on molecular similarities. The name "Spiralia" has been used to refer to this clade based on the presence of spiral cleavage during early development in many of its members. However, spiral cleavage occurs in some taxa not included in this group (e.g. some crustaceans), and does not occur in some within this clade (e.g. the Phoronids).
phyla in these this clade would have been grouped with other
phyla into three major groups, the acoelmates,
psuedocoelomates, and coelomates. As a way to
reconcile these basic body plans with the molecular data, it
has been proposed that acoelmate, psuedocoelomate, and
coelomate phyla evolved independently several times from an
ancestor that possessed all three coelom conditions
sequentially through life cycle (e.g. Rieger 1986).
I have represented
these three major groups as an unresolved trichotomy because
there relationships are still in question. Morphological
studies have traditionally considered the diploblast/radiata
phyla as a paraphyletic group (with cnidarians more closely
related to triploblast/bilateria phlya than to
poriferans). However molecular studies suggest the
cnidarians, poriferans, and ctenophores may be
monophyletic, though this may be an artifact of computer
analysis of molecular data know as 'long branch attraction (
Schmidt-Rhaesa,1998). In any case, molecular data
suggest that the Metazoa as a whole are a monophyletic group (Schutze,
"Platyzoa" is a
proposed name for the group of phyla that include
Platyhelminthes and Rotifers, but the name implies a
morphology (i.e. "flat") that is not representative of all
organisms in this grouping (e.g. Rotifers).
Despite gross morphological similarities to cnidarians and molecular evidence, some argue that ctenophores may be more closely related to the bilaterian animals than to cnidarians. This is based on the presence of anal pores, the third 'tissue layer' between the endoderm and ectoderm, a mixed bilateral and radial symmetry, presence of multicilated cells. Ctenophores are considered by some to be triploblastic as muscle fibers (derived from ameoboid cells) in the mesoglea. However, some molecular evidence suggests that ctenophores are more closely related to cnidarians than to the bilateral animals (Medina, et al. 2001)
Nemertines may be more closely related to phyla with coelomic cavities (specifically to Trochozoa ) based on recent molecular data. "Although the nemertean body plan is essentially acoelomate, the rhynchocoel is technically a coelom." Consequently, there has been debate about whether nemerteans are closely related to phyla lacking coelomic cavities. "A study using base sequences for the EF-1 gene also places Nemertea in Lophotrochozoa - in fact, within Mollusca." ( Harris 2002). Cavalier-Smith (1998) also places nemertines with Trochozoa phyla. According to Zrzavy et al (1998), the trochozoan affinity of the nemertines "is almost undisputable".
Some studies suggest
that mesozoans may be closer to Nematodes.
Cavalier-Smith (1998) consider mesozoans so different as to be
in their own subkingdom apart from their Subkingdoms Radiata,
Myxozoa, and Bilateria.
Though entoprocts are
morphologically similar to bryoans, development is strongly
protostome (whereas the lophophore
phyla display deuterstomic charartistics) and some
species develop into protostome-like trochophore
larvae. Like bryozoans, recent RNA data suggests
entoprocts are protostomes, though it is not clear how closely
they are related to the other lophophores (Aguinaldo et al.
1997) (some analyses suggest close relationship to
cycliophorans according to Pechenik 2000). A cladistic
analysis of 27 genera based on 20 characters supports a close
relationship between ectoprocts and entoprocts (Badorf
Entoproct larva from
Molluscs have been
considered by some to be an outgroup (less closely related) to
annelids and brachiopods based on Cambrian fossil evidence 7
Aplacophoran - considered by some to represent a more primative molluscan body form
may be related to rotifers (Garey
Schmidt-Rhaesa). Gastrotichs are
considered most closely related to gnathostomulids, and this
grouping most closely related to rotifers and acanthocephalans
by Cavalier-Smith (1998) based on shared characteristic that
include lack of segmentation, lack of vascular system,
ciliated, and no true coelom.
consider Myxozoa so different as to be in their own
subkingdom apart from their Subkingdoms Radiata, Mesozoa, and
Hemichordata may be
closer to Echinodermata than to Chordata as previous thought.
Pharyngeal slits and a dorsal, hollow nerve cord has been
considered as evidence for a sister-group relationship of
Hemichordata to Chordata rather than to Echinodermata. On the
other hand, the tornaria larva of enteropneusts resembles an
asteroid larva. "Recent studies using more 18S rDNA
sequences and a variety of analytical methods almost
invariably show Hemichordata to be monophyletic and more
closely related to Echinodermata than to Chordata" (Cameron,
Garey, and Swalla 2000; Halanych 1995; Turbeville, Schulz, and
Raff 1994 from Harris 2002).
"Symbion pandora was first collected in the 1960s from the mouthparts of the Norway lobster, but it was assumed to be a rotifer and stored in a museum drawer. The species was then rediscovered by Peter Funch andReinhardt Kristensen, who, after studying its many unique features and complex life cycle, erected the new phylum Cycliophora in 1995. Funch and Kristensen proposed that Cycliophora was close to Entoprocta and Ectoprocta. Comparisons of 18S rDNA suggest that Symbion is in Lophotrochozoa and is closer to Rotifera than to Entoprocta or Ectoprocta (Winnepenninckx, Backeljau, and Kristensen 1998)" (Harris 2002).
Recent major changes in phylogentic
relationship and therefore still controversial.
Traditionally platyhelminthes have been considered as an 'out group' from all the other triploblastic phyla (i.e. less related to these other phyla than these other phlya are to one another, or shared the last common ancestor with these phyla longer ago than any common ancestor shared among these other phyla). Platyhelminthes have more recently been represented as an unresolved trichotomy with lophotrochophores and rotifers based on one RNA study (Aguinaldo and Lake 1998 on p. 18 in Pechenik 2000) or represented as more closely related to rotifers and these two phyla more closely related to the lophotrochozoan phyla than to the other phyla (Cavailer and Smith 1998 on p. 27 in Barnes et al. 2001; and Garey and Schmidt-Rhaesa). According to Conway Morris (2000), "classically regarded as primitive triploblasts, the flatworms appear to be anatomically degererate, dispensing with such features as an anus".
However, there appears to be one group that may be the outgroup of the bilateria phyla (based on molecular evidence and supported by some morphological evidence): theTurbellarian order Acoels of the Platyhelminthes. If the Acoels are "the earliest branch within the bilaterian clade that left an extant ancestor", then this group will soon be considered as a seperate phylum (Ruiz-Trillo 1999).
The Phylum Annelida
now includes the formerly seperate phyla Pogonophora and
arthopods were considered more closely related to annelids and
molluscs (by virtue of being protostomes) than to to
deuterostomes such as chordates and echinoderms.
While the relationship among mollusc, annelid, and arthropod
groups was often debated (in part because both annelids and
arthropods are segmented), they were considered distinct from
the deuterostomes. However recent molecular data
(e.g. Zrzavy et al 1998) suggests that deuterstomes are more
closely related to molluscs and annelids (and the other
lophophorate clade) than to arthropods and their related phyla
(the ecdyzoa clade). Because this disagrees with
the traditional protostome grouping, I have represented these
three lines as an unresolved trichotomy.
At any rate, there is increasing
evidence that mollusc-annelid groups are not closely
related to ecdyzoa phyla (Schmidt-Rhaesa edt al. 1998).
Lophophorates exhibit a mix of deuterstome and protostome characteristics (see table below), though morphological and embyrological evidence leans toward deuterstomes. However, some recent molecular evidence suggests that the lophophorates are closely related to protostomes. This group may be more closely related to the Mollusc and Annelid group (collectively Lophotrochozoa;Garey and Schmidt-Rhaesa 1998) because they share unique Hox genes and because of 18S RNA evidence (the term Lophotrochozoa is derived from the phyla with lophophore and the phlya with trochophore larva). The text presents phyla in order that represent the traditional view (the lophophorates after the Arthropods) for perhaps two reasons. One, as stated above, the lophophorates show affinities to deuterstomes. Two, segmentation has traditionally been an arguement for close evolutionary relationship between annelids and arthropods.
Protostome Phoronids Brachiopods Bryozoans Deuterostome blastopore becomes: mouth mouth anus anus anus cleavage: spiral determinate radial indeterminate radial indeterminate radial indeterminate radial indeterminate coelom formation: schizocoelic schizocoelic (some) enterocoelous neither enterocoelous
considers tardigrades and onychophora more closely related to
one another than to arthropods based on shared characteristics
that include soft cuticle and unjointed limbs with terminal
(and the related minor phya, the nematomphorans, kinorhynchs,
locifera, and priapulids) were considered more closely related
to rotifers and acnthocephalans. Recent molecular
evidence indicates that the nematodes and related minor phya
are more closely related to arthropods, tardigrades, and
onychophorans (Aguinaldo et al. 1997). All these
phyla have the ability of individuals in all these phyla to
molt their cuticles (collectively the nematode-arthropod
phyla complex have been termed Ecdysozoa).
A previous cladistic analysis supported this clade as well
Explanations of phyla grouping terms
Unicellular organism exist as single cells or if colonial, cell differentiation is minimal. Protozoans comprise several uncellular phyla that are not included within the Kingdom Metazoa (All other "invertebrate" phyla in this course are within the Kingdom Metazoa). While many other organism are unicellular, at least one group of protozoans is thought to have given rise to metazoans. Protozoans are not a monophyletic group (e.g. Choanoflagellata and metazoans have a more recent common ancestor than other protists), but so that they are best defined as "eukaryotes that aren't plant, animal, or fungus."
are phyla with two well-organized tissue layers and are
radially symetrical. There is conflicting
evidence on whether this is a monophyletic group.
are phyla with three well-organized tissue layers and are
Gnathifera share a similar, complex jaw structure (and supported by molecular evidence)
possess a cuticle but do not molt (previously
considered related to nematodes and the related minor phya).
Rotifers and acanthocephlans are considered related because
both possess a syncytial epidermis and a peculiar stiffening
network of protein fibers (and is supported
These five phyla (the nematodes, nematomphorans, kinorhynchs, locifera, and priapulids) have brains that encircle the pharynx like a collar. Collectively they are known as Cycloneuralia. Cycloneuralia share a loss of locomotory cilia (Aquinaldo et al. 1997).
These three phyla (kinorhynchs, locifera, and priapulids) have have a spiny proboscis which can be everted (turned inside out) to gather food using the spines. Collectively they are known as Cephalorhyncha ("beak"-head).
comes from the names of the two major animal groups included:
the Lophophorata and the Trochozoa. Some
phylogenetic schemes include rotifers, platyhelminthes, and
related pyhla in the Lophotrochozoa group.
possess a lophophore, a crown-shaped (circular or U) feeding appendage
surrounding the mouth and bearing hollow (coelomic cavity)
tentacles. Water is pulled down the center of the
lophophore and this circulation is used for food-gathering and
gas exchange. However, the three lophophorata phyla
differ markedly with respect to circulatory and excretory
systems and in development.
Trochozoa share trochophore larval stage. Trochophore lava have two bands of cilia around the middle; at the "top" is a cluster of longer flagellae. However, the is controversy on the trochophore definition (Rouse, G. W. 1999).
build a cuticle, an outer layer of organic material that
functions as its skeleton and is flexible enough to function
as joints where this layer is thin. Many members
of this group regularly shed their cuticle, a process called
share molting cuticle (as in other ecdysozoans) and hemocoel
as well as other morphological characteristics.
Molecular data also support a close relationship among the
three phyla in this group. However, these groups share
many characteristics associated with other phyla (e.g.
Pechenik 2000; p. 402).
Hemichordata, Echinodermata, and Chordata as a monophyletic group of deutostomes appear to be withstanding molecular sequencing examination. Deuterstomes are characterized by:
The traditional grouping of phyla as protostomes does not appear to be monophyletic, and at least one analysis suggests the spiralians and dueterstomes have a more recent ancestor than do spiralans and ecdyzoans.
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