The Invertebrate Animals

Index to this page

The Origin and Evolution of Animals (Metazoa)

Invertebrate clade-rev4

Sponges (Phylum Porifera)

Sponges are

Cnidarians (Phylum Cnidaria)



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 (in most groups). Blastopore forms future anus. Mouth forms later.
Few HOX genes for the posterior Multiple HOX genes for the posterior
Spiral cleavage of Lophotrochozoan embryos Perpendicular cleavage planes in embryo [View]
Early cleavage cells committed [Link]; 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, Acorn worms, and Chordates

Let's first examine the protostomes. The deuterostomes are discussed below.

Lophotrochozoans vs. Ecdysozoans

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


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 (Phylum 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 (Phylum Annelida)

Characteristics: There are >15,000 species known. Some examples:

Mollusks (Phylum Mollusca)

With over 100,000 living species identified so far, 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, and cuttlefish, as well as the chambered nautilus. A record 28-foot (8.5 m) octopus and 60-foot (18 m) squid make these the largest of all the invertebrates.
  4. Scaphopoda. Marine, filter-feeding "tooth shells".
  5. 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).
  6. Polyplacophora. The animals in this group, called chitons, have their dorsal surface protected by 8 overlapping plates or "valves".
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 (Phylum Nematoda)


Arthropods (Phylum Arthropoda)

Some characteristics: We shall look at four groups (subphyla):


Hexapoda — the insects

Table listing some of the insect orders.


Some 13,000 species of (Neither group has the number of legs their name suggests, although one species of millipede does have 375 pairs.)



Evolutionary relationships of the arthropods

An ever-increasing number of arthropod gene sequences appear to have answered some long-standing questions about the evolutionary relationships of the various arthropod groups. A recent study (Regier, J. C., et al., Nature, 463:1079, 25 February 2010) examined 63 nuclear genes from 75 species of arthropods and concluded that

The Deuterostomes

In addition to the features listed above, the deuterostomes have (or had) gill slits. (The echinoderms have lost the gill slits of their ancestors.)

Echinoderms (Phylum Echinodermata)

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

Acorn Worms (Phylum Hemichordata)

The members of this small phylum (some 90 species have been identified) are marine forms most of whom live in burrows in ocean sediments.

Their closest living relatives are the echinoderms [View] with which they share the clade Ambulacraria. However, they possess a suite of features, both in their anatomy (e.g. gill slits) and their gene expression patterns, suggesting that their ancestors also led to the evolution of the chordates.

Chordates (Phylum 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:


This group (also called Tunicata) includes animals known as ascidians (and commonly called sea squirts). They are

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

It is hard to see what makes these animals chordates. The adults have neither notochord nor a dorsal tubular nervous system.

However, these animals disperse themselves with free-swimming larvae that have

(see the diagram above).

One of the most common species (Ciona intestinalis) has had its genome sequenced.

All these features are shared with C. elegans, but now we are talking about an animal far closer to the evolutionary line that produced us. In fact, with 80% of Ciona's genes having homologs in us, tunicates are probably our closest invertebrate relatives.


The representative member of this tiny subphylum of so-called lancelets is a small (5 cm), marine, fishlike creature called amphioxus (on the right). (For years its genus name was Amphioxus but that has now been replaced by the name Branchiostoma.)

Amphioxus 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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29 November 2023