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Feb 27, 2020 · Kingdom Animalia 101. The Kingdom Animalia is currently divided into 32 phyla (singular phylum). Generally speaking, people consider the animals of this world to be divided into two groups, the ‘vertebrates’ (those with backbones) and the ‘invertebrates’ (those without backbones).
The phyla Mollusca and Annelida belong to a clade called the Lophotrochozoa, which also includes the phylum Nemertea, or ribbon worms ( Figure 15.1.2 ). They are distinct from the Ecdysozoa (nematodes and arthropods) based on evidence from analysis of their DNA, which has changed our views of the relationships among invertebrates.
Even though members of the animal kingdom are incredibly diverse, most animals share certain features that distinguish them from organisms in other kingdoms. All animals are eukaryotic, multicellular organisms, and almost all animals have a complex tissue structure with differentiated and specialized tissues.
- Porifera. These are the simplest multicellular organisms that possess a cellular level of body organization, They are all aquatic, found mostly in marine habitats, and a few are freshwater forms.
- Cnidaria. These are mostly marine entities, while few, such as hydra, are found in freshwater. These are diploblastic and depict a tissue grade of organization.
- Platyhelminthes. They are bilaterally symmetrical, triploblastic, and acoelomate. They resemble leaves because of their dorsoventral flattening and lack of segmentation.
- Nematoda. Their body is triploblastic and bilaterally symmetrical. They demonstrate organization at the tissue level. They lack both the respiratory and circulatory systems.
Learning Objectives. Even though members of the animal kingdom are incredibly diverse, animals share common features that distinguish them from organisms in other kingdoms. All animals are eukaryotic, multicellular organisms, and almost all animals have specialized tissues. Most animals are motile, at least during certain life stages.
Circulatory system - Coelomates, Blood Vessels, Heart: Despite their greater potential complexity, many of the minor coelomate phyla (e.g., Pogonophora, Sipuncula, and Bryozoa) contain small animals that rely on direct diffusion and normal muscular activity to circulate the coelomic fluid.
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The phylum is further classified into five subphyla: Trilobitomorpha (trilobites), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustacea (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs).
