Kingdom Animalia includes all organisms that develop from a hollow
ball of cells called a blastula. Most animals have well-developed
motility.
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.
Coelenterates (phylum Cnidaria) are radially symmetrical, with two
tissue layers (ectoderm and endoderm) surrounding an all-purpose
gastrovascular cavity.
Phylum Porifera (sponges): Aquatic animals with radial symmetry or
irregular shapes. Water enters by many incurrent pores that
often 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.
Tissue-level organization: A tissue is a group of similar cells and their products,
built together (structurally integrated) and working together (functionally integrated).
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.
Sponges and Cnidaria
Phylum Cnidaria (coelenterates): Aquatic animals with two tissue layers
(outer ectoderm and inner endoderm) separated by a jelly-like
mesoglea; and an all-purpose gastrovascular cavity with a
single opening (mouth). Tentacles surround mouth and have
stinging cells (cnidocytes) containing stingers (nematocysts).
Two major body forms:
Polyp: mouth directed upward, mesoglea thin,
animal usually attached;
Medusa: free-swimming "jellyfish" with
thick 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.
Phylum Ctenophora ("comb jellies"): A small group of marine animals
with biradial symmetry (like a two-armed pinwheel), 2 large tentacles,
and 8 comb-like rows of cilia.
REVIEW:
Study guide and vocabulary
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