Kingdom Animalia is one of four kingdoms in the Domain Eukarya. It is distinct from the other three kingdoms, Plantae, Fungi, and Protista, in several ways. Animalia are multicellular, while most Protista (excepting the multicellular algae, which are plant-like) are unicellular. Heterotrophism separates the animals and fungi from plants, and the lack of cell walls in animal cells makes them distinct from fungi. Animals also possess several other unique features. These include interior digestion of food, possession of a digestive tract where hydrolytic enzymes are secreted and digestion takes place, and special cell junctions in their tissues.
The life cycle of organisms in Kingdom Animalia also separates them from organisms in the other three kingdoms. Animals spend their entire life cycle as diploid cells, with the exception of haploid gametes. The first stage of their life is as haploid reproductive cells (sperm and eggs) in the mature adult organisms. The gametes fuse to form a zygote. They zygote then undergoes mitotic divisions, which lead to a stage of development called the blastula. The blastocyst (blastula structure) consists of a single cell layer around a fluid-filled cavity. The formation of a gastrula, by infolding of the blastocyst in a blastopore, is also common to most animals. A gastrula consists of an inner and outer cell layer. The outer layer usually becomes the epidermal and nerve cells of the adult organisms--the ectoderm. The inner layer becomes the digestive tract, or endoderm. A third layer-the mesoderm-usually infolds, and develops into the other internal organs. From this stage, some animals develop into larva, which are immature specimens appearing very different from the adult. Larva then undergo a metamorphosis in which they become a mature adult, capable of reproducing.
Kingdom Animalia is thought to have arose in the sea, from colonial protists. It is believed that some of these protist colonies began to fold inward, creating a gastrula-like protoanimal. In this protoanimal stage, cell specialization occurred, paving the way for the evolution of true multicellularism. (See The Conclusions Essay for a much more thorough explanation).
The taxonomic system recognizes the generally accepted grouping of animals with certain evolutionary traits into taxa below the kingdom level, called phyla. The most well known phyla of kingdom Animalia are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, our own phylum. There are more than 35 phyla in all, but these nine generally comprise the bulk of the kingdom.
The radiation of diversity in the Animalia kingdom proceeded based on the evolution of a number of distinct "hallmark" features. The ancestral colonial protist that gave rise to the animal kingdom diverged first into two separate lineages葉he Parazoa (Latin for "false animal") and the Eumatazoa ("true animal"). The Parazoa contain only one modern phylum猶orifera, containing the sponges. The Parazoa are not true animals, per se, because they have no true tissues. In fact, a sponge can be passed through a sieve and reassemble on the other side with no damage to its overall cellular function. The Eumatazoa branch gave rise to all other modern animal phyla, those with true tissues.
The next division, a metaphoric "fork in the road" of evolution, occurred with the divergence of bilaterally symmetric animals from radially symmetric ones. Among the Eumatazoa, the phylum Cnidaria (hydras, jellies, coral polyps, and sea anemones) includes all of the radially symmetric animals葉hose that can be split into two identical halves by any cut made from top to bottom. Bilaterally symmetric animals, on the other hand, tend to swim or otherwise move around in a head-first direction. They are marked by a distinct posterior and anterior end, dorsal and ventral surfaces, and lateral surfaces. The bilaterally symmetric branch of the Eumatazoa gave rise to the rest of the animal phyla.
The next major split in the bilaterally-symmetric Eumatazoa occurred based on the evolution of body cavities (coeloms)庸luid filled spaces within the organism, excluding the digestive tract. Among the Eumatazoa phyla with bilateral symmetry, only the Platyhelminthes (the flatworms, including the subgroups planarians, flukes, and tapeworms) do not have any body cavity葉hey are the acoelomates. Rather, the flatworms have an incomplete alimentary canal surrounded immediately by endoderm, mesoderm and ectoderm tissue with no fluid-filled space in between. The first phylum to show any body cavity was the Nematoda, or roundworms. They are known as pseudocoelomates, for their body cavity is not a true coelom葉hat is, one surrounded entirely by mesoderm tissue. Rather, roundworms have a pseudocoelom between the endoderm and mesoderm tissue layers. It does provide some protection and a hydrostatic element for movement, but does not serve the same diversity of features as the true coelom. The true coelom, a fluid-filled space surrounded entirely by mesoderm tissue, is present in all of the remaining animal phyla葉he coelomates. It serves myriad protective and even skeletal functions, but perhaps its most important benefit is the ability to suspend organs from mesentary tissue and keep them stationary and cushioned from impact.
The final major division among the coelomates occurred with the divergence of the protostomes from the deuterostomes, manifest in several respects. In all coelomates, the alimentary canal is formed when the blastopore grows through the blastocyst structure. In protostomes, the blastopore becomes the mouth of the organism, while deuterostomes develop so that the blastopore becomes the anus. Protostomes have a coelom that develops from solid masses of mesoderm cells, while deuterostome coeloms develop instead from hollow outgrowths of the alimentary canal. Finally, protostomes exhibit determinate development from the beginning of the embryo, while deuterostomes show indeterminate development, taking up to 8 cell generations to differentiate. This means that if one were to take, say, a 16-cell morula of a protostome and a deuterostome and cut each into 16 separate pieces, each deuterostome cell would develop into a full organism. However, each protostome cell would develop only into a part of that organism. This accounts for the ability of scientists recently to clone 8 monkeys from a single embryo by splitting it; monkeys are deuterostomes. The protostome lineage includes the Annelida, Mollusca, and Arthropoda, while the deuterostomes include the Echinodermata and the Chordata.
These five phyla are distinct from each other on the basis of segmentation, quite a controversial evolutionary event. Among the protostomes, Annelida (the segmented worms, divided into earthworms, polychaetes, and leeches) and Arthropoda (the horseshoe crabs, arachnids, crustaceans, insects, centipedes, and millipedes) are segmented, while the Mollusca (snails, slugs, bivalves, squids, octopi) are not. The Arthropoda differ from the Annelida primarily in their possession of jointed appendages. On the deuterostome branch, the chordates are segmented, while Echinodermata are not. Evolutionary biologists continue to debate the phylogenetic origins of segmentation. Some believe it evolved separately in the protostome and deuterostome lineages after their divergence, while others believe it evolved very early and was later lost by the Echinoderms and Mollusks.
All of the organisms of Animalia are, perhaps contrary to the belief of most people, invertebrates. The only vertebrates are in the subphylum Vertebrata of the Phylum Chordata. Vertebrates are characterized by a nural crest, pronounced cephalization, a closed circulatory system, a skull, and backbone (spine) composed of multiple vertebrae.