Explanations and Difficulties in Invertebrate Phylogeny
for  a current probable phylogenetic tree of the invertebrates

Phylogenetic relationship undetermined due to significant contradictions of evidence or lack of evidence.

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).  

Traditionally various 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 ( Garey and Schmidt-Rhaesa,1998).  In any case, molecular data suggest that the Metazoa as a whole are a monophyletic group (Schutze, et al. 1998).


"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 2001).

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



Gnathostomulida may be related to rotifers (Garey and 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.



Cavalier-Smith (1998) consider Myxozoa  so different as to be in their own subkingdom apart from their Subkingdoms Radiata, Mesozoa, and Bilateria.


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 Echiura


Traditionally, the 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).

Cephalocarida- Hutchinsoniella



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



Cavalier-Smith (1998) 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 claws.



Previously nematodes (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 (Eernisse, 1992)


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."


Diploblasts/Radiata are phyla with two well-organized tissue layers and are radially symetrical.  There is conflicting evidence on whether this is a monophyletic group.


Triploblasts/Bilateria are phyla with three well-organized tissue layers and are bilaterally symetrical.

  Gnathifera share a similar, complex jaw structure (and supported by molecular evidence)

Syndermata 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 by molecular evidence)

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).



Lophotrochozoa 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.




Lophophorata 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 a band of cilia around the middle; at the "top" is a cluster of longer flagellae.  However, the is controversy on the trochophore definition (Rouse 1999).


Ecdysozoans 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 ecdysis.


Panarthropoda 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:

Traditionally, there has been serious debate as to whether several other phyla that posses some of these traits (the lophophorate phyla and the chaetognaths) should be included in the deuterostome clade.   Most molecular analyses do not support this.

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.


Some References:
Aguinaldo AMA, Turbeville JM, Linford LS, Rivera MC, Garey JR, Raff, RA, Lake JA.  1997.  Evidence for a clade of nematodes, arthropods and other moulting animals.  Nature 387: 489-493.


Cavalier-Smith, T. 1997a. A revised six-kingdom system of life. Biol. Rev. 73:203-266.  www.pnas.org/cgi/reprint/97/9/4426.pdf

Conway Morris, S. 2000.  The Cambrian “explosion�: Slow-fuse or megatonnage?  Proc Natl Acad Sci U S A. 2000 April 25; 97 (9): 4426­4429

Eernisse, D. J., J. S. Albert & F. E. Anderson. 1992. Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralian metazoan morphology. Systematic Biology 41(3): 305-330.

ANIMAL EVOLUTION.  In: Symposium on Evolutionary Relationships of Metazoan Phyla:
Advances, Problems and Approaches. Amer. Zool. 38:907-917.

Garey JR,   Schmidt-Rhaesa A, Near TJ,   Nadler SA.   1998.  The Evolutionary Relationships of Rotifers and Acanthocephalans.  From the Sixth Rotifer Symposium at Saint John's University.  Hydrobiologia 387-388: 83-91. http://chuma.cas.usf.edu/~garey/rotacanth.html

Harris CL.  2002. Teaching Animal Molecular Phylogenetics.

Medina, Monica ‚Allen G. Collins, Jeffrey D. Silberman, and Mitchell L. Sogin.  2001.    Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA.   Proc Natl Acad Sci U S A. 2001 August 14; 98 (17): 9707-9712.  http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=55517

Morris SC.  1999.  The Crucible of Creation: The Burgess Shale and the Rise of Animals.  Oxford University Press.  New York.

Pechenik. Jan A.  2000. Biology of the Invertebrates.  McGraw-Hill, New York.

Rieger RM. 1986. On the origin of the Bilateria: Significance of ultrastructural research for a new understanding of metazoan phylogeny. From http:/www.mesci.marine.edu/biology/labs/origin/default.htm

Rouse, G. W. 1999. Trochophore concepts: Ciliary bands and the evolution of larvae in spiralian Metazoa. Biological Journal of the Linnean Society 66: 411-464.

Ruiz-Trillo I, Ruitort M, Littlewood DTJ, Herniou EA, Baguna J.  1999.  Acoel flatworms; Earliest extant bilaterian metazoans, not member of platyhelminthes. Science 283: 1919-1923.

Schmidt-Rhaesa A, Ehlers U, Bartolomaeus T, Lemburg C,  Garey, JR.  1998.  The phylogenetic position of the Arthropoda.   http://chuma.cas.usf.edu/~garey/articulata.html

Schutze J., A. Krasko, M. R. Custodio, S. M. Efremova, I. M. Muller and W. E. G. Muller. 1999. Evolutionary relationships of Metazoa within the eukaryotes based on molecular data from Porifera. Proceedings of the Royal Society of London Series B 266:63-73.  http://acd.ufrj.br/labpor/4-Bibliografia/Pdfs/Schutze99.pdf

University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/phyla/metazoasy.html

Zrzavy et al.  1998.  (text fig 2.12 g of Pechenik 2000)