beardworm

▪ animal phylum

Introduction

also called  Pogonophoran,  

      any of a group of marine invertebrates constituting the phylum Pogonophora. Pogonophorans live a sedentary life in long, protective tubes on seafloors throughout the world. The common name beardworm refers to the beardlike mass of pinnate (feather-like) tentacles borne at the anterior end of many species. An intestine, which forms in embryos, disappears as development progresses. Males of the phylum are generally similar in appearance to females. Pogonophorans are the only multicellular animals that have neither mouth nor anus.

      Pogonophorans were first classified as a distinct phylum in the middle of the 20th century. The first species, Siboglinum weberi, described in 1914, came from the seas of the Malayan Archipelago; the second species, Lamellisabella zachsi, which came from the Okhotsk Sea, was described in 1933. In 1937 a new class called Pogonophora was established for Lamellisabella. In 1955 a close affinity between Siboglinum and Lamellisabella was proved, and the members were placed in the newly established phylum Pogonophora.

      Pogonophorans usually inhabit marine waters to depths that exceed 1,000 metres (3,280 feet); some species have been found at depths ranging from 7,000 to 10,000 metres. A native population inhabits the trenches of the Pacific and the Indian oceans. Many genera have a discontinuous distribution.

Natural history.

      The tentacles, probably used during feeding, vary in number according to body size. The tentacles are long processes containing blood vessels and are continuous with the body cavity, or coelom. Rows of very thin single-celled units called pinnules are found on the tentacles. The pinnules, which extend into the intertentacular cavity formed by the free or fused tentacles, intermesh to form a filter. Beside each pinnule base is a ciliary tract. In each intertentacular region, ciliary tracts produce a current of water that carries in microorganisms and other nutrients; these are filtered through the pinnule filter. Digestive enzymes probably are secreted by gland cells located at the pinnule bases. Water leaves by an opening at the base of certain tentacles. The digested food may be absorbed by the pinnules. The pinching off of small food particles in the outer layers of the pinnules during a process known as pinocytosis also has been observed. Direct penetration of amino acids from sea-water into the tentacles also has been reported to take place.

      Reproduction is sexual. Eggs, which have a high yolk content, are laid by the female in the anterior part of the tube, in which fertilization and early development to formation of a swimming larva also occur. Cleavage leads to the formation of a bilaterally symmetrical embryo. A cell layer called the endoderm first is represented by a few yolk-rich cells, which form a primary intestine; the intestine disappears as the yolk is utilized. Both mouth and anus fail to develop. A secondary body cavity is formed by separation of coelomic pockets from the primary intestine. The embryo develops into a larva having two ciliary belts and three body segments.

Form and function.

      The wormlike body varies in length from several centimetres to 0.5 metre (1.64 feet), the body diameter, from 0.06 millimetre to 4 millimetres (0.002 inch to 0.16 inch). Lamellibrachia barhami is one of the largest species. The body consists of three segments: two small anterior regions are called protosome and mesosome; the long trunk section is called the metasome. Each segment has its own coelom. The small protosome bears tentacles. The mesosome contains a structure known as a bridle, also called a frenulum, a pair of oblique cuticular ridges that extend backward to meet in the midventral line. The bridle supports the protruding worm on the edge of its tube. The metasome is divided into two sections by a pair of parallel ridges called belts; these have rows of small platelets containing minute teeth. One part of the metasome, in front of the belts, has a groove bordered by low folds.

      The protective tube is submerged in mud at the lower end; the very thin tubes of Sclerolinum, however, pierce pieces of sunken wood. The tube, secreted by special glands, contains a hard substance called chitin. Although the animals never leave their tubes, they are able to move inside them.

      The body wall consists of an outer layer of epidermal cells covered by a cuticle. Beneath the epidermal cells is a layer of longitudinal muscle cells. The nervous system, found within the epidermis, forms an almost continuous network. There are no sense organs. The circulatory system consists of two long vessels: in one, blood flows forward; in the other, backward. At the base of the tentacles, the vessel through which blood flows forward is differentiated into a muscular heart, which serves to push blood into the tentacle vessels. The blood contains the pigment hemoglobin. The excretory system consists of a pair of nephridia, or coelomoducts, which connect the body cavity with the exterior.

      The reproductive system consists of a pair of sexual glands that lie in the trunk. In females, ovaries are found in the anterior part of the metasome, while the sexual openings, or oviducts, are found in the middle of the body. In males, the testes are located in the posterior part of the body, and the sexual openings, called sperm ducts, in the anterior part. Numerous capsules containing sperm form in the sperm ducts of the males.

Evolution and classification.

      The Pogonophora may be separated into two classes, Afrenulata and Frenulata. The Afrenulata contains one species—Lamellibrachia barhami, which has been found in the Pacific Ocean near the coast of California. The class Frenulata contains 16 genera in six families. A total of 110 species of Pogonophora thus far have been identified; it is probable that many more species remain to be identified.

      According to general body plan and embryological development, Pogonophora should be included with the large group of animals that have a secondary body cavity; this group also includes Chaetognatha, Tentaculata, Hemichordata, and Echinodermata. A rich fossil Pogonophora fauna has been found in deposits of the Cambrian period (540 to 505 million years ago) and Ordovician period (505 to 438 million years ago) near St. Petersburg, in Poland, and in Siberia.

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