The simplest animals include the sponges (Porifera) and the Cnidaria.
Sponges are unsymmetrical or radiallysymmetrical, with many cell types but no distinct tissues; theirbodies contain numerous pores and sharp protective spicules.
Coelenterates (phylum Cnidaria) are radially symmetrical, with two tissue layers (ectoderm and endoderm) surrounding an all-purposegastrovascular cavity. Phylum Porifera (sponges): Aquatic animals with radial symmetry orirregular shapes. Water enters by many incurrent pores thatoften lead to a central cavity. Water may exit by an excurrent opening(osculum). No distinct tissues, but many cell types:
- Epidermal cells (pinacocytes): Outside lining
- Porocytes: Barrel-shaped pore cells
- Choanocytes: Flagellated "collar cells" that keep water flowing
- Amoebocytes: Several kinds of amoeboid cells, including some that secrete sharp spicules.
- Spicules: sharp needles or more complex shapes embedded within sponges, functioning in support and as a defense against predators. May be composed of silica, calcite, or horny protein.
You may think of a sponge as built of a single tissue.Types of sponges include:
- Calcispongia (or Calcarea): small sponges with needle-like spicules made of calcite (CaCO3). Symmetry usually radial.
- Hyalospongia (or Hexactinellida): deep-water "glassy" sponges (often very beautiful) with spicules (usually six-pointed) made of silica (SiO2). Symmetry usually radial.
- Demospongia: Horny or "true" sponges (about 90% of all known species) with spicules made mainly of horny protein ("spongin," similar to keratin) that often surrounds a needle-like core of silica. Irregular shapes (symmetry lost). Traditional "bath sponges" belong here.
Polyp: mouth directed upward, mesoglea thin, animal usually attached;
Medusa: free-swimming "jellyfish" withthick mesoglea; mouth directed downward.
- Class Hydrozoa: Life cycle includes both asexual polyps and sexually reproducing medusae (usually small). Solitary or colonial; some colonies have many types of individuals interconnected.
- Class Scyphozoa: Solitary "jellyfish" with dominant medusa stage.
- Class Anthozoa: The largest class, including sea anemones and corals. Polyp stage dominant; no medusa. Mouth extends inward to form a tubular pharynx. Solitary or colonial.
