PROTOSTOMATA

SYSTEMATIC BIOLOGY					KINGDOM ANIMALIA
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PHYLUM CHAETOGNATHA

INTRODUCTION TO THE CHAETOGNATHA

Chaetognatha (ke-TAG-na-tha) is made of two Greek roots meaning bristle jaws [bristle or mane -chaeto (χαίτη); and jaw -gnathos (γνάθος)]. The reference is to the bristles around the mouth of the arrow worm.

Chaetognaths or arrow worms superficially resemble tiny planktonic fish (Figure A). Brusca and Brusca (2003) and Margulis and Schwartz (1998) place the chaetognaths within the deuterostomes, but Nielsen (2001) does not. In fact, Nielsen (2001) argues that the arrow worms are more closely allied to the rotifers and gnathostomes, which would place them within the protostomes. Other treatments such as Tudge (2000) also consider them to be allied to the protostomes. At present, the arrow worms occupy a position of incertae sedis within the Paracoelomata.

A. Sagitta, a common planktonic chaetognath. It is small (8-12 mm long) and resembles a fish.

B. The anterior end of Sagitta. Note the spines and the large chamber associated with the head.

Images taken from:
A: http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/jpegs/sagit.jpg
B: http://www.vims.edu/bio/zooplankton/BATS/Chaetognatha/

SYNOPTIC DESCRIPTION OF THE PHYLUM CHAETOGNATHA

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), Colbert and Morales (1991), Nielsen (2001), and Tudge (2000).

I. SYNONYMS: arrow worms.

II. NUMBER: >100 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral; body divided into head, trunk, and post anal tail, all separated by septa.

Body Cavity: Eucoelomate; enterocoelic but without a peritoneum. One chamber in the head, two in the trunk, and one (+) in the tail.

Body Covering: Covered by thin chitinless cuticle.

Support: Hydrostatic skeleton.

Digestive System: Head with a large chamber bearing 4-14 large spines that help to grasp prey (Figure B). Simple tube with a muscular pharynx, two diverticula, and a ventral anus just anterior to the tail septum.

Circulatory System: Absent.

Locomotion: Active swimmer with nonmuscular fins.

Excretory System: Absent.

Nervous System: Circumpharyngial ring with a cerebral ganglion; anterioventral trunk ganglion and several lateral ganglia; sense organs include one pair of eyes, hair fans that detect vibrations and a ciliary loop on the dorsal region of the head and neck.

Endocrine System:

Respiratory System: Direct absorption through the skin..

B. Reproduction:

Reproductive System: Hermaphroditic. One pair of ovaries in front of the tail septum and a pair of testes behind the septum. Copulation occurs (self fertilization possible).

Development: Eggs cleave radially. Direct development.

C. Ecology: Free-swimming marine predators.O

HIERARCHICAL CLASSIFICATION OF THE CHAETOGNATHA

This taxonomy is taken from Brusca and Brusca (2003). The system has a single class (I call Chaetognathida) with 2 orders.

CLASS CHAETOGNATHIDA (2 ORDERS)

Gephyrospadella, Paraspadella, Spadella, Aberrospadella, Bathyspadella, Eukrohnia, Heterkrohnia, Krohnittella, Kukrohnia, Zahonya, Tokiokaspadella.

Bathybelos, Caecosagitta, Parasagitta, Sagitta, Pterosaggita, Krohnitta.

This page is maintained by Jack R. Holt. Last modified: 01/07/08.