| SYSTEMATIC BIOLOGY |
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| HOME | SYLLABUS | WEEKLY ASSIGNMENTS | J. SYSTEMATIC BIOLOGY | TAXA OF LIFE | |
| PHYLUM HEMICHORDATA | |||||
INTRODUCTION TO THE HEMICHORDATA
Hemichordata (he-mi-kor-DA-ta) is formed from two Greek roots that mean "half cord" [half -hemisu (ήμισυ); and cord -chordi (χορδή)]. The reference is to the presence of a notochord-like structure in the head of the animal.
The acorn worms are sluggish benthic marine animals with a head-like preoral region, a collar, and a worm-like body (Figure A). The preoral region has a stomochord, a notochord-like structure. The pterobranchs are sessile animals with lophophore-like structures that filter the water (Figure B). Other enigmatic taxa include Planktosphaera, a giant tornaria larva (Figure C). The graptolites are extinct animals that usually leave carbonized remains that resemble the the serrations of a saw blade. Likely, graptolites were early pterobranchs (Figure D).
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A. Balanoglossus, the acorn worm. |
B. Rhabdopleura, a pterobranch |
C. Planktosphaera, a giant tornaria larva of an unknown hemichordate. It is commonly found in marine plankton samples. |
D. Monograptus, an extinct colonial animal, likely a pterobranch. |
| Images taken from: A&D: http://www.ucmp.berkeley.edu/chordata/hemichordata.html B: http://cluster3.biosci.utexas.edu/faculty/cameronc/Images.htm#R.normani C. http://www.biology.duke.edu/johnsenlab/gallery/planktosphaera.html |
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SYNOPTIC DESCRIPTION OF THE HEMICHORDATA
| 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), Nielsen (2001), Colbert and Morales (1991), and Tudge (2000). |
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I. SYNONYMS: acorn worms. II. NUMBER: >100 species known. III. PHYLUM CHARACTERISTICS:
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SYSTEMATICS OF THE HEMICHORDATA
Tudge (2000) and Nielsen (2001) separate the acorn worms and pterobranchs into different unrelated phyla within the deuterostome line. I have kept them together in a single phylum after the systems of Brusca and Brusca (2003) and Margulis and Schwartz (1998).
HIERARCHICAL CLASSIFICATION OF THE HEMICHORDATA
| This is the taxonomic system of Brusca and Brusca (2003). The Class Planktosphaerida is problematic in that it likely is just a tornaria larva of an eneropneust. |
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CLASS ENTEROPNEUSTA (1 ORDER)
CLASS PTEROBRANCHIA (2 ORDERS)
CLASS PLANKTOSPHAERIDA (1 ORDER)
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This page is maintained by Jack R. Holt. Last revised 01/08/2008 .
