INTRODUCTION TO THE KINGDOM ANIMALIA

The Animal Kingdom  (Clade 1) is a natural, highly diverse group of organisms, most of which are multicellular and develop from a blastula (Margulis and Schwartz 1998).  However, I depart from Margulis and Schwartz (1998) in that I include the choanoflagellates and the myxozoans as part of the natural group.  At a deeper level, the Animal Kingdom (metazoans + choanoflagellates) is a sister group of the fungi (Bauldauf and Palmer, 1993), and together they form a group called the Opisthokonts (Cavalier-Smith and Chao 1995; Cavalier-Smith et al. 1996; and Patterson 1999).  This relationship has also been confirmed by supergroup analyses (Baldauf 2003a and Keeling 2004) which suggest a sister group relationship between the Opisthokonts and the Amoebozoa forming a larger group called the Unikonta.

The Choanozoan Clade (2): The Unicellular Animals

The choanoflagellates are unicellular or colonial organisms that are identical to sponge choanocytes in structure.  This relationship is more than superficial in that it has been confirmed by molecular evidence (Wainright et al. 1993; Cavalier-Smith et al. 1996).  Thus, the case for choanoflagellates as animals seems quite secure.  Indeed, Brusca and Brusca (2003), Nielsen (2001), and Tudge (2000) all indicate that the choanoflagellates are (at the very least) sister groups to the animal kingdom.  If the choanozoans are animals, then the Animal Kingdom grades from unicellular (Choanozoa) to multicellular (Metazoa) levels of structure.

The Metazoan Clade (3): The Multicellular Animals

The metazoa have cells organized into tissues and develop with an abbreviated life history (with a few derived exceptions) in which gonads produce gametes (eggs and sperm), the only haploid cells.   Nevertheless, some go through elaborate life cycles in which the individual may pass through a series of larval stages, some of which do not resemble the adult.  According to Adoutte et al. (2000), Conway Morris (1993), Nielsen (2001), Raff (2001), Anderson (2001a), Brusca and Brusca (2003), and Tudge (2000), the Metazoa has three somewhat unequal clades which I treat as subkingdoms that are defined according to their fundamental type of symmetry and level of cellular construction: the Parazoa (asymmetrical; tissue grade of construction), the Radiata (radially symmetrical; diploblastic level of construction), and the Bilateria (Prostomata and Deuterostomata; bilaterally symmetrical; triploblastic level of construction).  

The Parazoan Clade (4): The Tissue-Grade Animals

The parazoa include the sponges (Porifera) and an enigmatic group called the Placozoa.   Typically these animals are asymmetrical and develop through simple life histories.  Sponges can show a remarkable degree of cellular independence and survive and reassemble after separation of the cells.  Their construction as tissues of several cell-types, one of which is the choanocyte, makes them the simplest of the metazoa and connects them with the choanozoans.  Nielsen (2008) suggested that the higher-level complexities in the animal kingdom developed from the structure of a larval sponge that became sexually mature.  However, Adl et al. (2005) in an attempt to classify the Eukaryotes based on cladistic rules, separate the Animalia from the Porifera, Placozoa, and the Mesozoa and elevate all of them to the same rank.  Such a change goes against a long tradition of taxonomy and would require much more support to convince me at this point.  Furthermore, I am very skeptical about the separate or primitive natures of the Placozoa and the Mesozoa.  Indeed, the groups within the Mesozoa likely only bear superficial resemblance and have become secondarily simplified from a higher level of organization. 

Organ-Level Clade (5): Animals with Defined Organs and Symmetry

The Parazoan Clade is at the cellular or tissue-level of organization, but the Radiata + Bilateria have developed tissues as parts of organs and organ systems.  Furthermore, the organ-level animals tend to have determinate growth and develop along the prescribed lines of symmetry.  The cladistic analysis by Adl et al. (2005) concludes, among other things, that this (Clade 5) is the base of the Animal Clade and that the Parazoa and Choanozoa are sister groups to the animals.

The Radiate Clade (6): The Diploblastic Animals

Typically, these are the cnidarians (jellyfish, corals, and hydrozoans) and show a radial form of symmetry.  They are diploblastic, that is, they have two cells layers (endoderm and ectoderm) in mature animals.  Endoderm is the layer that lines the gut while the ectoderm is the cellular layer on the outside of the animal.  These layers also have developmental derivatives like the gonads.  The ctenophores, a jellyfish-like group, are tentatively placed together with the cnidarians, but may have become secondarily simplified.  A remarkable and fairly old hypothesis (Weil 1938, cited in Lom 1990) places the traditional “sporozoan” protozoa called the myxosporozoans (here called the Myxozoa) into the metazoans.  Weil suggested that the myxozoans evolved from free-living cnidarians and became extremely simplified as intracellular parasites (as have the narcomedusae, a group of parasitic cnidarians).  Indeed, the capsules of the myxosporidians bear a striking structural resemblance to the nematocysts of the cnidarians.  This view has slowly gained acceptance (e.g. Lom 1990; and Smothers et al.1994).  Smothers et al. (1994) confirm the structural association with molecular evidence that the myxosporidians are metazoans.

The Bilaterian Clade (7): The Triploblastic Animals

The bilaterians are, as the name implies, bilaterally symmetrical.  As they develop from the gastrula, a third cell layer, the mesoderm, develops between the ectoderm and endoderm.  Derivatives of the mesoderm provide much of the complexity seen in triploblastic animals.  For example, among vertebrates, the mesoderm develops into most of the bone, muscle, mesentaries, blood, etc.  Most of the vertebrate nervous system, as well as certain bones and other structures develop from the ectoderm.

Within the subkingdom Bilateria, I have kept the Protostomata - Deuterostomata dichotomy (these are approximately at the superphylum level).  The two groups differ fundamentally in how the gastrula develops.  The gastropore of a protostome (Clade 8) becomes the mouth while the gastropore of a deuterostome (Clade 9) becomes the anus.  Also, the protostomes develop by spiral cleavage and form a schizocoelic coelom.  The deuterostomes tend to develop by radial cleavage and form an endocoelic coelom.

The traditional taxonomic systems divide the bilaterian animals according to grades of body structure, especially within the Protostomata which is separated according to type of body cavity (i.e. the Acoelomates, the Pseudocoelomates, and the Eucoelomates).  Such a view can be seen in many pre-cladistic texts (e.g. Storer and Usinger, 1965) and even persist in more recent texts like Margulis and Schwartz (1998) and Nielsen (1995).  In a systematic sense, I have abandoned the old view of dividing the protostomes according to type of body cavity.  Cladistic analyses based on morphology and development (Brusca and Brusca, 2003; Nielsen, 2001) have led to the integration of acoelomate and pseudocoelomate taxa into the bilaterian clades.  

The deuterostomes and most of the protostomes are eucoelomate in structure, but some groups do not show clear affinities (i.e. the lophophorates and the chaetognaths).  Brusca and Brusca (2003), Nielsen (2001), and Margulis and Schwartz (1998) interpret the "lophophorates" as deuterostomes (although Nielsen says that the "bryozoans" are not related to the lophophorates and occupy a clade with the rotifers and gnathostomulids within the protostomes).  Furthermore, Nielsen (2001) and Brusca and Brusca (2003) placed the Annelida as a sister group to the panarthropods.  Other than the question of the position of the "bryozoan" phyla, they differ as to the position of the Chaetognatha.  Nielsen (2001) interprets chaetognath development and adult structure as being protostomal while Brusca and Brusca (2003) interpret the Chaetognatha as deuterostomal.  

The cycloneuralian hypothesis defines a protostome taxon, the Cycloneuralia, based on the presence of a ring of neural tissue that surrounds the upper esophagus and is promoted by Nielsen (2001) as well as Brusca and Brusca (2003).  The Cycloneuralia unites an eclectic group of taxa together: the Gastrotrichs, Nemata (Nematoda), Nematomorpha, Priapula, Kinorhyncha, and Loricifera.  Persons (personal communication) considers the Cycloneuralia "to be of dubious value" because the condition of having the anterior ganglion wrapped around the anterior gut is shared by about half of all bilaterians that have a complete gut.  He further points out that spiders, animals that show very few homologies with the cycloneuralians, also "have their brains wrapped around their esophagus and stomach but certainly wouldn't be included in this group."

Molecular phylogenetic trees suggest a very different organization of the animal kingdom than those generated by morphology and development.  Tudge (2000) summarizes current results of molecular phylogenetics in which he places the lophophorates in the protostomes and separates the protostomes into two fundamentally different lines: the Ecdysozoa and the Lophotrochozoa.  The Ecdysozoa grow by casting the external cuticle/exoskeleton and have similar introvert-like feeding organs (synapomorphies in this system).  The clade Lophotrochozoa has animals with lophophores or trochophore larvae as synapomorphies.  Besides the relationship implied by the molecular trees, the lophophorates and trochozoans appear to have no structural synapomorphies.   Raff (2001), in a summary of ribosomal DNA sequences, suggests a similar organization of the animal kingdom.  Valentine (2005) follows Giribet et al. (2000) whose analysis uses molecular, developmental, and structural evidence to suggest four bilaterian clades: Ecdysozoa, Platyzoa (called Paracoelomata by Valentine 2005), Lophotrochozoa (further separated into Eutrochozoa and Lophophorata), and Deuterostomata with the inscrutable acoel flatworms as sisters to all of the bilaterians.

The following system is a modification of Valentine (2005) and Giribet et al. (2000).  [A more detailed cladogram of the Bilaterians can be found at The Major Clades of the Animal Kingdom.]  In keeping with the systems of Margulis and Schwartz (1998) and Nielsen (2001), I have elevated the conventional subphyla of the "Chordates" to phylum-level.

HIGHER-LEVEL CLASSIFICATION OF THE ANIMAL KINGDOM

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