Phoronida

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Phoronida

(fərŏn`ədə), small phylum of slender, wormlike marine tube-dwellers, typically found in temperate, shallow seas. About 10 species are known. Although the body is free in the tube, the organisms extend only a crown of ciliated tentacles (the lophophore) to capture food. Water currents generated by the lophophore cilia sweep food particles against mucus secreted at the base of the tentacles and ciliated grooves propel the food to the mouth. Phoronids have a U-shaped digestive tract, a blood-vascular system containing hemoglobin, and excretory organs called metanephridia. The coelom, or body cavity, is divided into compartments resembling those seen in the Ectoprocta and the Brachiopoda, which are phyla related to the Phoronida; the compartments also resemble those of the Echinodermata. Phoronids are ancient, and some zoologists have suggested they are the ancestors of the brachiopods because of similarities in embryology. Tubes seen in early paleozoic sandstones appear to be identical with modern phoronid tubes, but little is known of their history.

Phoronida

A small, relatively homogeneous group of animals now generally considered to constitute a separate animal phylum. Two genera, Phoronis and Phoronopsis, and about 16 species are recognized.

 Phoronopsis harmeri removed from its tubeenlarge picture
Phoronopsis harmeri removed from its tube

Phoronids may occur in vertical tubes placed just below the surface in intertidal or subtidal mud flats, or as feltlike masses of intertwined tubes attached to rocks, pilings, or old logs in shallow water. In both cases the tubes, composed basically of a secreted, parchmentlike material, are encrusted with small particles of sand or shell. A third living habit concerns those phoronids found inside channels, probably self-made, in limestone rock or the shells of dead pelecypod mollusks.

The geographical distribution of phoronids appears to be worldwide in temperate and tropical seas. There are no records of phoronids from the polar regions.

The body is more or less elongate, ranging in length from about 1.6 to 8 in. (4 to 20 cm), and bears a crown of tentacles arranged in a double row surrounding the mouth which is usually crescent-shaped (see illustration). The anus occurs at the level of the mouth and is borne on a papilla immediately outside the double row of tentacles. The digestive tract is therefore U-shaped, the mouth and anus opening close together at one end of the animal. The tentacles rest on a connective tissue base known as the lophophore. Associated with the mouth is a ciliated flap of tissue known as the epistome. See Lophophore

The phylum includes both dioecious animals and hermaphrodites. All phoronids may reproduce sexually, and in most cases the life history includes the pelagic actinotroch larva. Some species reproduce asexually by transverse fission.

Phoronida

[fə′rän·ə·də]
(invertebrate zoology)
A small, homogeneous group, or phylum, of animals having an elongate body, a crown of tentacles surrounding the mouth, and the anus occurring at the level of the mouth.
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References in periodicals archive ?
In particular, phoronid transcriptomes contained a number of TLR contigs with at least 6 putative homologs recovered for Phoronis vancouverensis and 4 for Phoronis psammophila (Table 3).
None of the examined shells showed evidence of the presence of phoronids in the outer face.
This work constitutes the first report of the presence of phoronids of the genus Phoronis forming tubes, in the shell of the red abalone Haliotis rufescens.
Phoronids and brachiopods have different levels of functional and developmental integration of their larval and juvenile tissues among species (detailed in Santagata and Zimmer, 2002), but in general, the juvenile neuromuscular system is developed precociously within the body of the larva, and neuronal connections are shared between the larval nervous system (apical ganglion and tentacular nerve rings) and the juvenile neuromuscular system.
Species of phoronid are also noted for their wide geographical distributions and often occur in conspecific aggregations (Emig, 1982; Zimmer, 1991).
7 to 3 times longer to develop to swimming than the phoronid and brachiopod with uniciliated cells.
Equivalent patterns of neuropeptide localization in other marine larvae have been reported for the molluscan neuropeptide FMRFamide, which is also found in central larval ganglia and peripheral nerves in larval phoronids (Hay-Schmidt, 1990).
The phoronid actinotrocha differs strikingly from both spiralian and deuterostome larval forms in its large, mobile, and muscular oral hood, present throughout growth and development of the larva [ILLUSTRATION FOR FIGURE 1 OMITTED].
1991; Kempf and Page, 1995), the prosobranch Haliotis (Barlow and Truman, 1992), phoronids (Hay-Schmidt, 1990), polychaetes (Hay-Schmidt, 1995), brachiopods (Hay-Schmidt, 1992) and various echinoderms (Moss el al.
1992), brachiopods (Hay-Schmidt, 1992), phoronids (Hay-Schmidt, 1990b,c), a hemichordate (Dautov and Nezlin, 1992), and a cephalochordate (Holland and Holland, 1993) - less is known of the roles of these compounds in the mediation or modulation of larval behaviors.
Several workers have postulated that the lophophores of brachiopods, bryozoans, and phoronids are homologous to the tentaculate arms of pterobranch hemichordates (3-8), leading to phylogenetic hypotheses that support the notion of homology among these structures (8, 9).
Phoronid neighbors (Johnson 1990) and the design of bryozoan colonies (Lidgard 1985) also have been shown potentially to enhance filter-feeding success.