The Invertebrate Animals

Index to this page

The Origin and Evolution of Animals (Metazoa)

Sponges (Phylum Porifera)

Sponges are

Cnidarians (Cnidaria)

External Link
Site devoted to the Cnidaria with illustrations
(Please let me know by e-mail if you find a broken link in my pages.)


All the remaining groups of animals belong in a clade whose members share: The bilaterians contain two clades, the protostomia and the deuterostomia.

Protostomia vs. Deuterostomia

Long before the days of genome analysis, taxonomists were convinced of a fundamental division in the animal kingdom between the
Protostomia Deuterostomia
Blastopore forms future mouth and anus Blastopore forms future anus only. Mouth forms later.
Few HOX genes for the posterior Multiple HOX genes for the posterior
Spiral cleavage of embryo Perpendicular cleavage planes in embryo [View]
Early cleavage cells committed; no identical twins Early cleavage cells totipotent; identical twins possible
Coelom arises by splitting of mesoderm Coelom arises between invaginating mesoderm during gastrulation
Lophotrochozoans and Ecdysozoans Echinoderms and Chordates

Lophotrochozoans vs. Ecdysozoans

Genome analysis, especially the analysis of supports a major division of the Protostomia:


Their name was created from the names of formerly-separated groups that have now been joined in a single clade on the basis of the similarities of their genomes:

The clade contains a number of phyla of which we shall examine only 3.

Flatworms (Platyhelminthes)

This phylum contains some 20,000 species distributed among three classes:
Link to page illustrating the life cycles of the fish and pig tapeworms.

Annelids (Annelida)

Characteristics: There are about 12,000 species known, distributed among the

Mollusks (Mollusca)

With 70,000 living species (perhaps only a third of the number out there), the mollusks must be counted as among the most successful animals on earth today. Most belong to the first 3 of the 6 classes shown here:
  1. Bivalvia. Two shells encase the body. Includes the clams, mussels, oysters, and scallops.
  2. Gastropoda. Snails and slugs. Snails have a single shell ("univalves') while slugs have none.
  3. Cephalopoda. This marine group includes the various species of octopus, squid, as well as the chambered nautilus. A record 28-foot octopus and 60-foot (18 m) squid make these the largest of all the invertebrates.
  4. Scaphopoda. Marine, filter-feeding "tooth shells".
  5. Amphineura. Includes the chitons, whose shell is made of 8 overlapping plates ("polyplacophora")
  6. Monoplacophora. Until a live specimen was discovered in 1952, these animals were thought to have been extinct for millions of years. It has a single shell (hence the name) and, unlike the other mollusks, is segmented (as are its relatives the annelids).
The trochophore larvae of mollusks is also evidence that they belong in the same clade with the annelids.
Link to drawings of representative mollusks (132K)


All the members of this clade The clade includes a number of phyla of which we shall examine 2:

Roundworms (Nematoda)


Arthropods (Arthropoda)

Some characteristics: We shall look at:



Link to drawings of representative crustaceans (112K)


The Deuterostomes

The features of these animals are listed above. The group comprises the echinoderms and the chordates.

Echinoderms (Echinodermata)

Characteristics: There are 5 classes of echinoderms:
Link to drawings of representative echinoderms and the water vascular system (100K)

Chordates (Chordata)

During their embryonic development, all chordates pass through a stage called the pharyngula [View] with these features: The vast majority of chordates have a skull enclosing their brain (Craniata), and all but one of these (the hagfish) convert their notochord into a vertebral column or backbone. These latter are the vertebrates.

Vertebrates also differ from all the other animals by having quadrupled their HOX gene cluster; that is, vertebrates have 4 clusters of HOX genes located on 4 different chromosomes.

The vertebrates are described in a separate page. Link to it.

Here we shall examine two groups of invertebrate chordates, the:


This is a group of

The one on the right is Halocynthia, the sea peach (photo courtesy of Ralph Buchsbaum).

Aside from the gill slits, it is hard to see what makes these animals chordates. The adults have neither notochord nor dorsal tubular nervous system.

However, these animals disperse themselves with free-swimming larvae that have both notochord and a dorsal tubular nervous system (see the diagram above).


The representative member of this tiny subphylum is a small (5 cm), marine, fishlike creature, the lancelet (Amphioxus, on the right).

It retains: throughout its life.

There is a small cluster of neurons at the anterior tip of the nerve cord with certain similarities of structure and gene expression to the vertebrate fore-, mid- and hindbrain.

Although able to swim, the lancelet spends most of its time partially buried in the sand while it filters microscopic food particles from the water.
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3 August 2001