Heterotrophic Invertebrates: Symmetry, Phylum Porifera, and Cnidarians

Heterotrophic Invertebrates

Heterotrophic organisms cannot produce their own food. They ingest their food. There are different types of symmetry.

Symmetry in Organisms

Symmetry is when one shape becomes exactly like another at any position. Symmetry can be divided into three groups:

• Asymmetrical: Organisms that lack symmetry and therefore are sessile, having no definite shape.
• Radially Symmetrical: Organisms that can be divided into similar halves by passing a plane at any angle along a central axis.
• Bilaterally Symmetrical: A body plan in which the left and right sides of the organism can be divided into approximate mirror images of each other.

Invertebrates

Invertebrates are organisms that do not have a backbone. 98% of animals are invertebrates. They are ectotherms, or cold-blooded, meaning they warm their bodies by absorbing heat from their surroundings.

Phylum Porifera: Sponges

Sponges evolved over 500 million years ago. They are asymmetrical, having no symmetry. Sponges have two outer layers separated by a gel layer called mesophyl. In the gel layer are spongin fibers.

Types of Sponges

Sponges come in two basic types:

• Encrusting: Cover rocks like mosses.
• Free-standing: Grow to large proportions.

Sponges are filter feeders, able to capture and eat particles as small as bacteria, as well as much larger particles. Food is collected in choanocytes.

Reproduction in Sponges

• Asexually: Through budding or fragmentation.
• Sexually: Most sponges are hermaphrodites.

Phylum Cnidaria: Simple Invertebrates

Cnidarians are simple invertebrates with tissue level organization and radial symmetry. They have two main body plans: polyp or medusa.

• Medusa: Mouth facing down; the second stage of the life cycle.
• Polyp: Tube-like, like a vase with the mouth upward; can be solitary or part of a colony.

Characteristics of Cnidarians

• Nematocysts: Specialized cells that contain toxins to paralyze or poison prey.
• Gastrovascular cavity: Where digestion occurs.
• Epidermis: Tissue around. Cells enter directly by diffusion.
• Oxygen from water enters by osmosis.
• Excretion occurs by diffusion out of the water.

Reproduction in Cnidarians

• Asexually: By budding.
• Sexually: In this stage, there is an alternation of generations.

Classes of Cnidarians

• Anthozoa: Sea anemones and coral. All Anthozoa are in the polyp stage.
• Hydrozoans: Include hydras and the Portuguese Man-of-War (Physalia). The polyp stage is dominant. The Portuguese Man-of-War's venom can kill a person.
• Scyphozoans: Jellyfish. They have a dominant medusa stage and are food for larger animals.

Most Dangerous Cnidarian

The Australian box jelly is considered one of the most dangerous animals in the world.