INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 3 - H.Y. Shcherbak - 1997

PHYLUM POGONOPHORA

The phylum Pogonophora, which comprises bizarre marine creatures, was established only in the mid-20th century, and research on it continues to this day.

These animals have been found in almost all seas and oceans of our planet with normal Water salinity, although until quite recently they were considered rare creatures occurring only at great depths. More than 150 species have been described.

Pogonophorans inhabit the sea floor inside chitinous tubes that are open at both ends and exhibit diverse structures: ranging from delicate, parchment-like and elastic to mud-dwelling and rigid ones. Throughout their lives, they never leave their tubes, yet can move freely up and down within them. The length of the tubes exceeds the body length, sometimes by a factor of two. The substance forming the tube is secreted by numerous multicellular glands that open through ducts on the body surface.

Most known species of pogonophorans are colorless or whitish and translucent. Blood Vessels shine through the body integuments, particularly the tentacles, giving the tentacles a bright red color.

The body of pogonophorans is thread- or cord-like and cylindrical, with its length exceeding its thickness by 100–500 times. The body consists of four regions that differ somewhat in Structure among the Representatives of the classes Frenulata and Afrenulata (or Vestimentifera) included in the phylum; however, in all pogonophorans, the first region bears tentacles (ranging from one or two to many thousands), hence the name of the phylum — Pogonophora, meaning "beard-bearers".

The body, including the tentacles, is covered by a cuticle under which lies a single-layered epithelium with numerous unicellular glands. Beneath the epithelium is a layer of circular Muscles and, deeper still, longitudinal muscles; the contraction of either Muscle layer causes strong elongation or shortening of the body.

On the inside, the muscles are lined with peritoneal epithelium, which bounds a well-developed secondary body cavity (coelom).

Unlike most free-living Multicellular animals, adult pogonophorans completely lack a Digestive System. They feed on METABOLISM/21.html">Chemosynthesis products, owing to a Symbiosis with sulfur-oxidizing Bacteria, or by absorbing Amino Acids directly from seawater. Excretory Organs, when present, are represented by coelomoducts located in the first body region.

The Circulatory system is well developed and closed, with an orderly blood flow. As already mentioned, the blood is red due to the presence of Hemoglobin.

There are no specialized respiratory organs; gas exchange occurs primarily through the tentacles.

The Nervous system is quite primitive. It lies entirely within the thickness of the Skin epithelium and is represented by a nerve plexus featuring concentrations of Nerve Cells and fibers — the so-called Brain, located on the ventral side of the first body region, which innervates the tentacles — and a longitudinal nerve cord extending along the entire body.

Pogonophorans are dioecious, although Sexual Dimorphism has not been detected in them. Reproduction and embryonic development have been studied only in representatives of the Class Frenulata (see p. 198).

Pogonophorans are a quite ancient group of animals; their tubes, described under the name Sabelitida, are known from the Late Precambrian.

Regarding The system of pogonophorans, different views exist; however, most specialists recognize two classes within this phylum: Frenulata and Afrenulata (or Vestimentifera).