Defined as multicellular animals that do not possess a vertebral column, an internal skeleton or a cranium, invertebrates represent 95% of all multicellular animal species on Earth; though unicellular life first evolved in primordial seas about 3.6 billion years ago, marine invertebrates did not appear until 800 million years ago (near the end of the Precambrian Era). Vertebrates are grouped within one phyla (Chordata) and are represented by about 66,000 species; by contrast, invertebrates are classified within 33 phyla and are represented by more than 1.3 million species.
The largest invertebrate phylum is Arthropoda, which includes more than 1.1 million species of insects, spiders and crustaceans. However, 14 invertebrate phyla are represented by various types of worms; the largest of these is Nematoda, which includes more than 25,000 species of roundworms. Among the other invertebrate phyla are Porifera (sponges), Cnidaria (anemones, jellyfish, corals), Mollusca (slugs, snails, squid, clams, octopus ) and Echinodermata (starfish, sand dollars, sea urchins).
The study of invertebrates certainly makes one appreciate the magnificent diversity of life on Earth. Once we become familiar with all of the invertebrate species, we can move on to unicellular organisms, fungi, plants and vertebrates (of which we are a member species). In total, we know of more than 1.7 million species of life on our planet (at least 3/4 of which are invertebrates).
Correction: In the initial post, crustaceans were incorrectly listed with Mollusca; in fact, they are Arthropods.