Invertebrates

Invertebrates are animals without a backbone or vertebral column. Vertebral column is made up of small bones called vertebrae.

The scientific name assigned to all animals without backbones is invertebrate, meaning without vertebrae

This course covers

• characteristics of invertebrates and classify them.
• distinguish between invertebrates and vertebrates.
• characteristics, classification, and life cycles of
  • Protozoa
  • Porifera
  • Coelentrata
  • Platyhelminthes
  • Nematoda
  • Arthropoda
  • Annelida
  • Mollusca
  • Echinodermata

Invertebrates are ectotherms (cold-blooded). Most invertebrates live in water or spend at least some part of their life in water.

Body symmetry is one of the important characteristics of the invertebrtaes; cnidarians have radial symmetrical bodies, while arthropods have bilateral symmetrical bodies.

Classification of Invertebrates

Invertebrates are categorized into eight major categories, called phyla (Figure 1).

Phylum Protozoa includes microscopic unicellular eukaryotes like amoeba, paramecium.

Phylum Porifera contain animals with pores bodies known as sponges.

Phylum Coelenterata, also called Cnidaria includes animals such as hydra, jellyfish, sea anemone, and corals. These animals are characterized by having special stinging organs called nematocysts.

Phylum Platyhelminthes includes flatworms like tapeworms.

Phylum Nematoda includes bilaterally symmetrical round worms like hookworm and pinworm.

Phylum Arthropoda includes animals with exoskeleton such as insects, crustacean, arachnid, myriapod.

Phylum Annelida includes segmented worm such as ringworm, earthworm, and leeches.

Phylum Mollusca include snails, slugs, scallop, octopus, and clams. They are called mollusks and are have soft body and shell.

Phylum Echinodermata consist of a group of animals with external spines, such as starfish, sea urchin, and sea cucumber.

Table showing differences and similarities between Invertebrates and Vertebrates

Characteristics	Invertebrates	Vertebrates
Kingdom	Animalia	Animalia
Phylum	Chordata	Chordata
Symmetry	Bilateral and radial	Bilateral
Physical	Multicellular, no backbone, no cell wall	Multicellular, backbone present, Advanced brain, neurons and integumentary system
Nutrition	Parasitic and heterotrophic	Heterotrophic
Size	Small and slow moving	Big
Heart and circulatory system	Dorsal heart, open circulatory system	Ventral heart, closed blood vessel system
Reproduction	Asexual and sexual	Sexual
Species	Constitute 98% of the animal species	Constitute 2% of the animal species
Examples	Sponges, insects, worms	Fish, snakes, birds, mammals