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PHYLUM GNATHOSTOMOZOA

INTRODUCTION TO THE GNATHOSTOMOZOA

Gnathostomozoa (NATH-o-stom-o-ZO-a) is made of three Greek roots meaning jaw-mouthed animals [jaw -gnathos (γνάθος); mouth -stoma (στόμα); animal -zoa (ζώο)]. The reference is to the jaw-like structures in the pharynx.

The gnathostomes are a small group of acoelomates that share characters with pseudocolelomates and eucoelomates. Indeed, Nielsen (2001) claims that these are part of the clade that includes the polychaete worms. Clearly, their incomplete digestive tract with a degenerate anus, absence of excretory structures, etc. all suggest that they are highly modified and reduced from a coelomate form. Giribet et al. (2000) grouped them together with taxa both eucoelomate and pseudocoelomate, thet they called the Platyzoa. Valentine (2004) placed them in the clade that he called the Paracoelomata.

A. A light micrograph of Gnathusomula.

B. An SEM micrograph of the "jaws" of Gnathostomula.

Images taken from:
http://www.zmuc.dk/InverWeb/Dyr/Limnognathia/phylogeny/Gnathostomulida_UK.htm

SYNOPTIC DESCRIPTION OF THE PHYLUM GNATHOSTOMOZOA

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

I. SYNONYMS: gnathostomes.

II. NUMBER: >80 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral

Body Cavity: Some disagreement. May be present but highly reduced. Organ systems in parenchymatous mesoderm.

Body Covering: Ciliated epithelium (Figure A). Each epidermal cell with only one flagellum.

Support: Hydrostatic skeleton.

Digestive System: Blind, tubular gut. Muscular pharynx with paired jaws (Figure B) and hard comb-like basal plate.

Circulatory System: Absent.

Locomotion: Ciliated epithelium and muscular contractions.

Excretory System: ?

Nervous System: Some concentration of nervous tissue toward the anterior end.

Endocrine System: None.

B. Reproduction:

Reproductive System: Males, females and hermaphrodites. Internal fertilization. Oviparous.

Development: Zygote develops by spiral cleavage; no larval stage.

C. Ecology: Marine, inhabit interstitial and detrital habitats.

HIERARCHICAL CLASSIFICATION OF THE GNATHOSTOMOZOA

Taxonomy of the Phylum follow the taxonomic system of Brusca and Brusca. However, because of confusion with their name for the phylum (Gnathostomulida; Brusca and Brusca, 2003) and a typical class name, I have changed the phylum name to Gnathostomozoa.

This phylum has a single class (CLASS GNATHOSTOMULIDA) and two orders.

Gnathostomula, Problognathia, Haplognathia, Pterognathia, Austrognathia.

This page maintained by Jack R. Holt. Last modified: 01/05/08