DEUTEROSTOMES

SYSTEMATIC BIOLOGY					RETURN TO THE DEUTEROSTOMATA
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY	TAXA OF LIFE
PHYLUM UROCHORDATA

INTRODUCTION TO THE UROCHORDATA

Urochordata (u-ro-kor-DA-ta) is made of two Greek roots that mean "tail cord" [tail -ura (ουρά); and cord -chordi (χορδή)]. The reference is to the notochord being restricted to the tail of the larval "tadpole".

The sea squirts generally are benthic or sessile marine filter-feeders. As adults they appear almost sponge-like with incurrent siphon, a large filtering pharynx with gill slits and an out current siphon (Figures A&B). However, larval forms are shaped like tadpoles with a notochord in their tails. They also have a hollow dorsal nerve chord and pharyngial gill slits, all hallmarks of the "chordate" taxa. Some species (the Appendicularia) retain the larval form throughout their lives and remain active swimmers in the plankton (Figure C).

A. A group of living tunicates. The sponge-like nature is evident. Note the siphons.

B. A preserved Molgula.

C. Oikopleura, a larvacean, so called because it retains its larval form. The animal (center) secretes a thin mucilaginous "house" around itself with which it filters food particles.

Images taken from:
A: http://www.ucmp.berkeley.edu/chordata/urochordata.html
B: The Systematics Biodiversity Collection
C: http://oceanexplorer.noaa.gov/explorations/02arctic/background/fauna/media/oik_lab.html

SYNOPTIC DESCRIPTION OF THE UROCHORDATA

The following description of the phylum Urochordata came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Nielsen (2001), Storer and Usinger (1965), Colbert and Morales (1991), Ruppert et al. (2004), and Tudge (2000).

I. SYNONYMS: tunicates, sea squirts.

II. NUMBER: 2150

II. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral.

Body Cavity: Eucoelomate but body cavity lost in adults.

Body Covering: Covered encased in external fibrous case or tunic, often of cellulose.

Support: Hydrostatic skeleton (notochord in larval forms only).

Digestive System: Large pharynx with many gill slits (stigmata) for filter-feeding; food trapped in mucus; food tube short and simple, often U-shaped with anus near atrial siphon.

Circulatory System: Blood system with sinuses in mesenchyme; heart reverses periodically.

Locomotion: Usually adults are sessile.

Excretory System: Absent.

Nervous System: Simple in adults; ganglion between siphons from which nerves issue directly.

Endocrine System:

Respiratory System Absorption through pharynx.

B. Reproduction:

Reproductive System: Hermaphroditic; often with one ovary and one testis. External fertilization. Asexual reproduction common.

Development: Embryo forms tadpole larva with muscular postanal tail, notochord and hollow dorsal nerve cord, usually tail reabsorbed when animal matures.

C. Ecology: Sessile benthic or planktonic marine animals; filter-feeders, one group carniverous.

SYSTEMATICS OF THE UROCHORDATA

I have followed the system of Nielsen (2001) in raising the tunicates to phylum-level status.

HIERARCHICAL CLASSIFICATION OF THE UROCHORDATA

This taxonomy is taken from the system of Brusca and Brusca (2003). However, the elevation of the the tunicates from a subphylum of Chordata to its own phylum is from Nielsen (2001). The phylum has four classes.

CLASS ASCIDIACEA (4 ORDERS)

Sessile; branchial and atrial siphons close together; life-cycle simple, without alternation of sexual and asexual generations.

Ascidia, Botryllus, Chelyosoma, Ciona, Clavelina, Corella, Diazona, Diplosoma, Halocynthia, Lissoclinum, Molgula, Polycarpa, Psammascidia, Pyura, Styela.

CLASS THALIACEA (3 ORDERS)

Planktonic; branchial and atrial siphons at opposite ends of barrel-shaped or fusiform body; exhalant water current provides propulsive force for movement; either colonial or alternate between solitary and colonial forms; asexual buds formed on ventral stolon.

Dolioletta, Doliolum, Pyrosoma, Salpa, Thetys.

CLASS APPENDICULARIA (ALSO CALLED LARVACEA, 1 ORDER)

Small, solitary; neotenous; with large tail; secrete gelatinous house through filters of which water is drawn by beating movements of the tail; when the house is clogged, it is discarded and a new one is secreted; pharynx with 2 stigmata opening directly to body surface; endostyle reduced; tail with notochord and nerve cord, much larger than body.

Fritillaria, Oikopleura, Stegasoma.

CLASS SORBERACEA (1 ORDER)

Ascidian-like, but carnivorous, no perforated branchial sac. They keep a dorsal nerve cord as adults. Abyssal.

Octacnemus.

This page is maintained by Jack R. Holt. Last modified 01/08/2008 .