DESCRIPTION OF THE PHYLUM HEMICHORDATA (BATESON 1885)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>HEMICHORDATA |
PHYLUM HEMICHORDATA LINKS
| 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 name of this group was coined by Bateson (1885) and usually is considered to be a subphylum of the Chordata. |
| INTRODUCTION TO THE HEMICHORDATA The acorn worms are sluggish benthic marine animals with a head-like preoral region, a collar, and a worm-like body. The preoral region has a stomochord, a notochord-like structure, which is a dorsal extension of the pharynx (Pechenik 2005). Hemichordates are similar to the chordate deuterostomes in that they have gill slits (perforations of the body wall through which water may pass). In addition, they have a short hollow nerve cord associated with the collar. Despite their similarities to the chordates, both morphological and molecular work show a sister relationship with the echinoderms (Cameron et al. 2000, Bourlat et al. 2006, Delsuc et al. 2006 and Putnam et al. 2008). This suggests that the occurrence of gill slits and a hollow nerve cord might be primitive in the group and, therefore, undermines part of the morphological basis for the chordate concept. The hemichordates fall into two major groups: Enteropneusta and Pterobranchia (Figure 1). Tudge (2000) and Nielsen (2001) separate the enteropneusta (acorn worms) and pterobranchs into different unrelated phyla within the deuterostome line. We have kept them together in a single phylum after the systems of Brusca and Brusca (2003), Margulis and Schwartz (1998), Pechenik (2005), and Ruppert et al. (2004). The analysis of Halanych (1995) confirmed the relationship between the enteropneusts and the pterobranchs with the further observation that the hemichordates are more closely related to the echinoderms than they are to the chordate phyla (Cephalachordata, Urochordata, and Craniata). Swalla and Smith (2008) summarized the relationships of deuterostome taxa using molecular, morphological and paleontological characters. Interpretations of all analyses of these groups suggest that the characters of hollow dorsal nerve chord and ciliated gill slits must be plesiomorphic in the Deuterostomata. |
FIGURE 1. Cladogram showing the relationship of the major groups of Hemichordata (in the colored box) with other deuterostomes (members of clade D). The hemichordates are sisters to the echinoderms in a clade called Ambulacria (clade A). B = Bilateria.
| The Enteropneusta The enteropneusts are the acorn worms (Figure 2), which are worm-like and mobile. They can grow up to 2.5m long. The acorn worms have a distinctive head and collar that make them look like a worm wearing an acorn on its anterior end. Their coelomic cavities are highly reduced. The gut is straight from the mouth, located at the base of the preoral bulb or proboscis, and extends to the end of the long trunk. They can regenerate themselves (also an attribute of the echinoderms) and undergo asexual reproduction. In the sexual life cycle, the fertilized egg develops into a planktonic tornaria larva (text with tooltip) A tornaria larva is a characteristic larval form of hemichordates. It is planktonic, transparent, and long-lived. , which provides dispersal before settling down to develop into the typical acorn worm. Typically, they are benthic animals as adults and inhabit U-shaped tubes. They pull water through the tube and collect detritus by ciliary action on the proboscis which carries trapped particles to the mouth. |
| The Pterobranchs Pterobranchs (Figure 2) are very different from the enteropneusts. They are sessile and colonial with zoids growing in a common, branching tube. The tentacles emerge from the collar and function to capture suspended organic matter and carry it by ciliary action to the mouth. A colony of zoids have bodies that are connected by a long thin tissue called a stolon. The gut, however, is U-shaped and not confluent with the other zoids. The ciliated tentacles of the pterobranchs resemble the lophophores of the Bryozoa, Brachiopoda, and Phoronida, one of the reasons that the lophophorates were once considered part of the deuterostomes. |
| The Planktosphaerids Some enigmatic taxa are found in this phylum. Planktosphaera is a giant free-living tornaria larva (Figure 3) likely with affinities with the Enteropneusta. Though that has not been confirmed. At present, they are given uncertain status as a class (Planktosphaerida) equal to the enteropneusts and petrobranchs. |
| The Graptolites The graptolites were Paleozoic animals that usually left carbonized remains, resembling the the serrations of a saw blade. They were colonial and appeared to be free-floating. Likely, graptolites were early colonial pterobranchs (Figure 4). |
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| FIGURE 2. Balanoglossus, the acorn worm. Typically, the animal would be in a U-shaped tube in the substrate. Image from: http://www.ucmp.berkeley.edu/chordata/hemichordata.html | FIGURE 4. Rhabdopleura, a pterobranch, a colonial organism of filter-feeding zoids. Image from: http://cluster3.biosci.utexas.edu/faculty/cameronc/Images.htm#R.normani | FIGURE 5. Monograptus, an extinct colonial animal, likely a pterobranch. Image from: http://www.ucmp.berkeley.edu/chordata/hemichordata.html |
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| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013 |
