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PHYLUM PLACOZOA

INTRODUCTION TO THE PLACOZOA

Placozoa (plak-o-ZO-a) is a combination of two Greek roots that mean flat animal [plax, a flat tablet or table (plax); and zoa, an animal (ζώο)].  This is a reference to the flattened appearance of the animal.

The Placozoa is a small group of simple organisms with very little structural complexity, a grade that Brusca and Brusca (2003) call the 'Mesozoan Grade of Complexity".  The upper and lower surfaces of this irregular and flattened animal are made of ciliated epithelial cells that envelop a loosely-organized mesenchymal layer (See Figures A&B above).  As an animal, it is very odd.  The mitochondria are tubular and the extracellular matrix has no collagen (Adl et al. 2005).

Trichoplax adhaerens.

A.  Labeled drawing of Trichoplax, the single genus in this phylum.

Placozoan from Guam, roughly 400 microns across

B. Low power photomicrograph of Trichoplax.
Images taken from:
A: http://www.ldeo.columbia.edu/edu/dees/ees/life/slides/phyla/placozoa.html
B: http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html

SYNOPTIC DESCRIPTION OF THE PLACOZOA

The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Nielsen (2001), Storer and Usinger (1965), and Tudge (2000).
 

I. SYNONYMS: Placozoa was defined by Grell in 1971.  It has no common synonyms.

II. NUMBER: 1 or 2 known species.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: None.

Body Cavity: Not present.

Body Covering: Upper surface covered by ciliated squamous epithelium; lower surface covered by columnar ciliated epithelium.

Support: Hydrostatic by fluid-filled cavity.

Digestive System: Saprobic (Extracelluar, extracorporeal). Enzymes released from lower surface.

Circulatory System: None.

Locomotion: Movement by ventral ciliated epithelium.  The ventral cells have a single flagellum with two orthogonal basal bodies (kinetosomes).

Excretory System: None.

Nervous System: None.

Endocrine System: None.

B. Reproduction:

Reproductive System: No special organs. Eggs and non-flagellated sperm are produced. Asexual reproduction occurs.

Development: Not known.

C. Ecology: Found mainly in marine aquaria.

SYSTEMATICS OF THE PLACOZOA

According to Barnes (1980) members of the group, particularly Trichoplax, resemble the planula larva of the Radiata.  Thus, the group may represent either an ancestral complexity commensurate with their appearance or a derived, neotenic form.  Either way, the animal has such a reduced genome that it approximates that of a bacterium (Brusca and Brusca 2003).  Adl et al. (2005) treat the Placozoa as a group equivalent to the Porifera, Mesozoa, and the Animalia.  I consider it as a group of uncertain status at the parazoan grade of organization.

HIERARCHICAL CLASSIFICATION OF THE PLACOZOA

This taxonomy is from the systems of Brusca and Brusca (2003) and Margulis and Schwartz (1998).
 

The taxonomy is uncertain, and the phylum has only 1 or maybe 2 species in a single genus.

Trichoplax.

This page is maintained by Jack R. Holt.  Last modified: 03/14/08