The Animal Kingdom 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.  The case for the choanoflagellates 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.  The Choanoflagellates 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, in this system the Animal Kingdom grades from unicellular (Choanozoa) to multicellular (Metazoa) levels of structure.

A remarkable and fairly old hypothesis (Weill 1938) places the traditional “sporozoan” protozoa called the myxosporozoans (here called the Myxozoa) into the metazoans.  Weill (1938) 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 Animal Kingdom (metazoans and choanoflagellates) are sister groups of the fungi (Bauldauf and Palmer, 1993) and together form a group called the Opisthokonts (Cavalier-Smith and Chao 1995; Cavalier-Smith 1996; and Patterson 1999).  This relationship has also been confirmed by supergroup analyses (Baldauf 2003 and Keeling 2004) which suggest a sister group relationship between the Opisthokonts and the Amoebozoa forming a larger group called the Unikonta.

According to Adoutte et al. (2000), Conway (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; cellular grade of construction), the Radiata (radially symmetrical; diploblastic level of construction), and the Bilateria (Prostomata and Deuterostomata; bilaterally symmetrical; triploblastic level of construction).  See Major Clades of the Animal Kingdom for a summary of the relationships.    All groups have cells organized into tissues and organs 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.

Within the subkingdom Bilateria, I have kept the Protostomata - Deuterostomata dichotomy (these are approximately at the superphylum level).  However, the traditional taxonomic system divides 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 (1996).  In a systematic sense, I have abandoned the old view of dividing the bilaterially symmetrical animals 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.  Typically the protostomes develop by spiral cleavage and form a schizocoelic coelom.  The deuterostomes tend to develop by radial cleavage and form an endocoelic coelom.

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).  Nielsen (2001) as well as Brusca and Brusca (2003) unite the Gastrotrichs, Nemata (Nematoda), Nematomorpha, Priapula, Kinorhyncha, and Loricifera into a group that they call Cycloneuralia (a reference to the ring of neural tissue that surrounds the upper esophagus in this group).  Also, they have 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 their development and adult structure as being protostomal while Brusca and Brusca (2003) interpret the Chaetognatha as deuterostomal.  

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 cycloneuralian groups, 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 (2004) follows Giribet et al. (2000) whose analysis uses molecular, developmental, and structural evidence to suggest four bilaterian clades: Ecdysozoa, Platyzoa (called Paracoelomata by Valentine 2004), Lophotrochozoa (further separated into Eutrochozoa and Lophophorata), and Deuterostomata with the enigmatic acoel flatworms as sisters to all of the bilaterians.

Adl et al. (2005) in a recent 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, anyway. 

The following system is a modification of Valentine (2004) and Giribet (2000).  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.

REFERENCES

This page maintained by Jack R. Holt.  Last modified: 04/22/08