The Animalia Animal kingdom is a multicellular eukaryotic group characterised by heterotrophic nutritional mode, usually organ and tissue development, and motility sometime during the organism's life history. Cnidarians are almost all marine carnivores and are characterized by alternation between polyp and medusa. They also have a gastrovascular cavity with one opening that is both the mouth and anus. Nematodes , commonly called roundworms, live in all aquatic, terrestrial, and parasitic environments and often occur in great numbers.
Roundworms have a complete digestive tract with a mouth and anus.
Mollusks are soft-bodied animals with a specialized layel of epidermal cells called a mantle that secretes a shell. Annelids include earthworms, leeches, and many less familiar marine and freshwater species. The most distinctive characteristic of this phylum is segmentation. Phylum Arthropoda is the largest phylum of animals and includes spiders, ticks, mites, scorpions, centipedes, millipedes, shrimp, crabs, and insects. The external skeleton, or exoskeleton is made of chitin, which is a long chain of nitrogen-containing sugar molecules.