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Animals - phyla (in development)

Authors: Even Dankowicz

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Depending on who you ask, there are about 30 phyla of animals. No disagreement exists regarding the status of common groups like chordates, molluscs, or echinoderms, but several uncommon and microscopic groups are alternatively placed in their own phylum or with others. The ordering in this document is primarily based on the phylogenetic tree at tolweb.org.

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Porifera (sponges)

Sponges are sessile, asymmetrical marine and freshwater invertebrates.

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Placozoa

Placozoa are simply organized, lacking clear body symmetry. Placozoa are found in warm ocean waters, mostly near shore.

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Ctenophora

Ctenophora are radially symmetrical. Ctenophora, or comb jellies, feed on plankton in marine environments.

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Cnidaria

Cnidarians are radially symmetrical. Cnidarians usually have stinging tentacles. Corals are a type of sessile cnidarian which form large colonies. Cnidarians are found in marine and freshwater aquatic habitats.

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Brachiopoda

With paired shells, brachiopods could easily be confused with bivalve molluscs, but unlike bivalves, the two shells of brachiopods are not entirely symmetrical. Brachiopods are marine.

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Phoronida (horseshoe worms)

Phoronida are worm-like marine animals that secrete rigid tubes around their bodies, in sand or attached to a shell or rock. A ring or spiral of tentacles are arranged around the mouth to collect food--some Annelid tubeworms are similar, but have feathery, not thread-like, appendages around the mouth.

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Entoprocta

Entoprocta are small, marine animals, attaching to rocks or other substrates and often forming colonies, which somewhat resemble sessile cnidarians.

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Nemertea (ribbon worms)

Nemerteans are mostly marine, but some freshwater and terrestrial species exist. These worm-like animals are unsegmented and most species are somewhat flattened.

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Mollusca

Molluscs are diverse soft-bodied animals including snails, clams, and squid, found in marine, freshwater, and terrestrial habitats. Many molluscs possess distinctive shells for protection.

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Sipuncula (peanut worms)

Sipuncula are unsegmented worms found in diverse marine habitats.

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Bryozoa

Bryozoans are sessile, freshwater or marine organisms that mostly form mat-like colonies on rocks, shells, and other surfaces.

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Annelida (segmented worms)

Annelid worms are found in marine, freshwater, and terrestrial habitats, including familiar forms such as earthworms and leeches. Annelid worms are divided into distinct body segments, which sometimes bear paddle-like appendages.

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Platyhelminthes (flatworms)

Flatworms are found in marine, freshwater, and terrestrial habitats; many are parasitic or predatory.

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Mesozoa

Mesozoa contain both the Orthonectida and the Rhombozoa/Dicyemida, both of which are sometimes considered independent phyla. Mesozoa are microscopic parasites of marine invertebrates.

Wikimedia / public domain

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Cycliophora

Cycliophorans are usually found in the mouthparts of lobsters, where they feed on leftover food and bacteria. None are more than half a millimeter in length. Cycliophorans are only known from oceans in the Northern hemisphere.

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Micrognathozoa

A single microscopic species belongs to this phylum, Limnognathia maerski, from a cold spring in Greenland.

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Gnathostomulida (jaw worms)

Gnathostomulida are microscopic unsegmented worms. They are found in marine environments, mostly in shallow, nutrient-rich, oxygen-deprived areas of the ocean floor.

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Rotifera (wheel animals)

Rotifers are mostly found in marine and freshwater habitats, although a few are terrestrial. Rotifers are microscopic, with a variety of geometrically distinctive body shapes. The Acanthocephala are sometimes considered a separate phylum. Some rotifers, such as at the right, build themselves a tube-like shelter.

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Gastrotricha (hairy-bellied worms)

Gastrotricha are microscopic worm-like animals found in marine and freshwater habitats. The flattened underside of the body is covered with cilia.

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Chaetognatha (arrow worms)

Chaetognatha are marine worms found in various marine habitats, and range from microscopic to 12 cm in size.

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Priapulida

Priapulid worms are found in relatively shallow mud along ocean shores.

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Loricifera

Loricifera are microscopic creatures found in the marine seafloor.

Wikimedia / public domain

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Kinorhyncha

Kinorhyncha, or mud dragons, are microscopic seafloor creatures found in marine or coastal habitats, where they feed on organic matter.

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Nematoda (roundworms)

Nematodes are simple and worm-like, without distinct appendages or any visible segmentation. Most species are microscopic, but a few grow larger. They are abundant in marine and terrestrial habitats.

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Nematomorpha (horsehair worms)

Nematomorpha are mostly marine, but some are found in freshwater or damp soil. The longest species are a meter long, but most are closer to 10 cm. Nematomorpha are similar to nematodes in appearance, and much like some nematodes are parasites of arthropods.

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Tardigrada (water bears)

Tardigrades are microscopic animals with four pairs of legs found in marine, freshwater, and terrestrial habitats.

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Onychophora (velvet worms)

Velvet worms are soft-bodied, with paired conical legs and antennae. Velvet worms are found in terrestrial habitats of Central and South America, Africa, and Australasia, where they are predators of other invertebrates.

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Arthropoda

Arthropods make up more than half of all animal species, in almost any imaginable marine or terrestrial habitat. Arthropod bodies are divided into jointed segments which are typically armored with a rigid external exoskeleton. Most arthropods have paired eyes and at least one pair of antennae. Members of the arthropods and chordates are the only animals with the ability to fly.

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Echinodermata

Adult echinoderms are radially symmetrical. Echinoderms are only found in marine environments, and include starfish, sea urchins, and others.

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Hemichordata

Hemichordates are closely related to echinoderms and chordates, and share gill slits and some aspects of body organization with the latter. The most familiar hemichordates are those known as acorn worms; a second group is similar but forms colonial groups. All hemichordates are marine.

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Chordata

Chordates all possess a notochord, a rod passing along the length of the body which is developed further into the backbone in vertebrates. Most chordates are vertebrates, which are distinct from other animals in possessing a bony or cartilaginous vertebral column. The other main group of chordates are the sessile, marine, tunicates, shown in the bottom right. Chordates are widespread in aquatic and terrestrial habitats.

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Sources

Introduction to the Hemichordata, https://ucmp.berkeley.edu/chordata/hemichordata.html

Chang et al, 2015. Genomic insights into the evolutionary origin of Myxozoa within Cnidaria. PNAS 112(48): 14912-14917
Kristensen, Funch, 2000. Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida. Journal of Morphology, 246(1): 1-49
Sterrer, 2013. Gnathostomulida (Unsegmented Marine Worms). Wiley Online Library. https://doi.org/10.1002/9780470015902.a0001587.pub3