Only sponges (phylum Porifera) have asymmetrical body plans. Some animals start life with one type of body symmetry, but develop a different type as adults; for example, sea stars are classified as bilaterally symmetrical even though their adult forms are radially symmetrical.
Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial, or five-sided, symmetry, echinoderm larvae are ciliated, free-swimming organisms that organize in bilateral symmetry which makes them look like embryonic chordates.
Many species have a calcareous shell. Molluscs are bilaterally symmetrical, although some have secondarily lost the symmetry in part; they have well-developed excretory, digestive, respiratory and circulatory systems. In all molluscs except the cephalopods, the circulatory system is open.
They are radially symmetric, diploblastic animals and coral is of the basic polyp form meaning that it is a sessile animal that attaches the base of it's body to a surface, with it's mouth and tentacles facing upwards.
: symmetry in which similar anatomical parts are arranged on opposite sides of a median axis so that only one plane can divide the individual into essentially identical halves.
Like the flatworms, nematodes are bilaterally symmetrical. They take their name from their round body cross-sectional shape. Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system.
The flatworms, like the sponges (Phylum Porifera) and Phylum Cnidaria (Hydra, jellyfish, corals, and sea anemones) are multicellular. Unlike, the animals in the Phylum Cnidaria which have radial symmetry, those in the Phylum Platyhelminthes have bilateral symmetry.
The Ctenophora have partial radial symmetry like the Cnidaria, and partially bilateral symmetry (two similar sides) like the bilateral animals (most other animals.).
The simplest animals include the sponges (Porifera) and the Cnidaria. Sponges are unsymmetrical or radially symmetrical, with many cell types but no distinct tissues; their bodies contain numerous pores and sharp protective spicules.
Radial symmetry is advantageous to sedentary organisms because sensory receptors are evenly distributed around the body. For sessile (sedentary) organisms, radial symmetry is useful because the animal can just "sit down" and grab food or detect threats from all directions.
In this respect, arthropods are built like humans are; the right half of an arthropod is a mirror image of its left half — this is called bilateral symmetry (bi = two, latus = side). That is called radial symmetry.
The phylum Cnidaria (sea anemones, corals, hydras and jellyfish) is the likely sister group of the triploblastic Bilateria. Cnidarians are generally regarded as diploblastic animals, possessing endoderm and ectoderm, but lacking mesoderm.
There are two basic body plans in cnidarians. They are called the polyp and medusa.
Cnidaria do not have a brain or groups of nerve cells ("ganglia"). The nervous system is a decentralized network ('nerve net'), with one or two nets present. They do not have a head, but they have a mouth, surrounded by a crown of tentacles. The tentacles are covered with stinging cells (nematocysts).
Cnidarians are said to be the simplest organisms at the tissue grade of organization; their cells are organized into true tissues. Cnidarians are essentially bags made of two cell layers. The outer ectoderm, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum.
Cnidaria are some of the simplest and most beautiful "animals." These creatures are flower-like and resemble plants in many respects. However, they have a mouth and a simple digestive system at the center of their tentacles. Because of these two features, these creatures are considered animals and not plants.
What are three characteristics that all cnidarian have in common? Cnidarians have an epidermis, gastrodermis, mesoglea, gastrovascular activity and tentacles. Also, they have cnidocytes and a nervous system composed of diffuse web of interconnected nerve cells called a nerve net.
Common features of cnidarians include radially symmetrical diploblasts with true tissues and nematocysts. The presence of nematocysts indicates that all cnidarians arose from a common ancestor. The major evolutionary step that occurred with the phylum cnidaria was the development of tissue-level organization.
Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved.
All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the hypostome, surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey.
