Invertebrate Anatomy OnLine
Phoronis architecta ©
Phoronid
5jul2006
Copyright 2001 by
Richard Fox
Lander University
Preface
This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises, a glossary, and chapters on supplies and laboratory techniques are also available at this site. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.
Systematics
Lophophorata SP, Phoronida P, (Fig 25-35, 9-26)
Lophophorata SP
Lophophorata includes three taxa, Phoronida, Bryozoa, and Brachiopoda, sharing several morphological characteristics. Some zoologists include Kamptozoa in this group. The first three taxa possess a funnel-shaped anterior ring of ciliated tentacles known as a lophophore (Fig 25-2, 25-25A). The lophophore surrounds the mouth and is an upstream collecting system for suspension feeding. Its tentacles are hollow with extensions of a coelomic space thought to be a mesocoel. The gut is U-shaped with the anterior mouth at the center of the lophophore. The anus is also anterior, but is dorsal to the mouth, outside the ring of the lophophore (Fig 25-2A).
The lophophoral tentacles bear two types of cilia. Frontal cilia on the inside face of the tentacles extend into the interior of the lophophore, whereas lateral cilia on the sides of the tentacles extend into the gap between adjacent tentacles. The mouth is at the bottom (apex) of the lophophore funnel and is encircled by the ring of tentacles. The feeding current is generated by the lateral cilia. In lophophorates and many filter feeding deuterostomes (but not Kamptozoa) water enters the open end of the lophophore, moves toward the mouth, and then exits laterally between the tentacles (Fig 25-25A). Food particles are captured on the upstream side of the tentacles and transported to the mouth by the frontal cilia. In cross section the lophophore may be horseshoe-shaped or circular (Fig 25-4).
Lophophorates inhabit a secreted enclosure, tube, shell, or zooecium, that may be organic or mineral. The body is divided into two parts, the mesosome and metasome, each with a coelomic space. The small mesosome is the region of the lophophore and the much larger metasome is the trunk and accounts for most of the body. The tiny anterior epistome is sometimes considered to be a third body region homologous to the protosome of early deuterostomes. Lophophorates are suspension feeders and most are marine but some occur in freshwater.
Phoronida P
Phoronida is a small taxon of 14 worm-shaped lophophorates. They are benthic, infaunal or epifaunal, marine suspension feeders. When conditions are favorable they may be present in high population densities (Fig 25-1). They secrete and inhabit a stiff chitinous tube and have a horseshoe-shaped (usually) or circular lophophore (Fig 25-2). Sand, shell fragments, debris, sponge spicules, and other particles may adhere to the chitinous tube.
The body consists of two regions, each containing a coelomic space (Fig 25-2B). The mesosome with its mesocoel is larger and is the region of the lophophore. It surrounds the mouth as a coelomic ring with extensions into the tentacles of the lophophore. Most of the body is the metasome, or trunk, and its cavity is the metacoel, or perivisceral coelom, or the trunk coelom. The metacoel is partitioned by four longitudinal mesenteries into four compartments (Fig 25-5). A third region, the small anterior epistome overhangs the mouth.
The long gut is complete and extends from the anterior, terminal mouth to the subterminal, also anterior, anus situated on the dorsal midline near the anterior end and just above the mouth. The closed hemal system contains hemoglobin in corpuscles. The lophophore is the major respiratory structure. Two metanephridia open through nephridiopores beside the anus (Fig 25-2B). The nervous system includes a nerve ring at the anterior end and a longitudinal nerve cord on the left side (usually).
Phoronids may be gonochoric but most are hermaphroditic. Fertilization is internal. The larva is an actinotroch, a type of trochophore. Phoronids often undergo asexual reproduction which may result in aggregations of individuals. The individuals in the aggregation may be entangled with each other.
Laboratory Specimens
Phoronids are not often available in teaching laboratories but under some circumstances sufficient numbers may be available, typically for courses offered at coastal locations. The exercise is written for Phoronis architecta but other large phoronids can also be used with little deviation from these descriptions. Phoronis architecta inhabits shallow (high subtidal) silty sand bottoms and constructs a straight, sandy, chitinous tube about 10 cm in length and about 1-2 mm in diameter.
Living material should be studied initially in clean sea water and then moved to isotonic magnesium chloride. A dissecting microscope is required for the study.
External Anatomy
Examine the anterior end of a living animal with the dissecting microscope. The mesosome is the extreme anteriormost of the two phoronid body regions. It bears the lophophore and mouth and contains the mesocoel, which extends into the tentacles of the lophophore (Fig 25-B). Find the lophophore and note that its tentacles are arranged in a horseshoe shape, rather than a simple circle (Fig 25-4B). The mouth is at the bottom (apex) of the lophophore encircled by the ring of tentacles. The feeding current is generated by the lateral tentacular cilia and food particles are transported to the mouth by the frontal cilia. Water enters the open end of the lophophore and exits laterally between the tentacles and out of the lophophore (Fig 25-25A). Food particles in the exiting current strike the tentacles and initiate a local reversal of beat in the lateral cilia on the sides of the tentacles which throws them back into the bell. This may happen several times before they eventually strike the frontal cilia on the inside edge of the tentacles and are moved by them toward the mouth. There are no cilia on the outside edges of the tentacles.
>1a. Visualize the pattern of current flow by watching the movement of small suspended particles in the water. If necessary add a suspension of carmine particles seawater to enhance the visualization. Does the motion of particles through the lophophore of your animal conform to the description above? <
Relax the animal in isotonic magnesium chloride and then carefully remove it from its tube. What effect does magnesium chloride have on the cilia? What effect does it have on the shape of the lophophore? Does the funnel collapse? The tentacles are held erect by hydrostatic pressure in the mesocoel which is generated by body wall muscles. Muscles are inactivated by magnesium but cilia are not affected.
Begin your study of the worm using the lophophore to orient yourself. It is located ventrally on the anterior end of the animal. Use a pair of minuten nadeln on applicator sticks (see Techniques chapter) to spread its tentacles apart and look for the crescent-shaped mouth on the midline inside the lophophore (Fig 25-2B). The mouth is displaced to the ventral side of the anterior end and is surrounded by tentacles.
The epistome forms a ridge overhanging the mouth dorsally (Fig 25-3B). It contains a space that some believe is homologous to the protocoel of the early deuterostomes.
Ventral to the epistome is a ciliated groove that conveys food particles from the bases of the tentacles to the mouth. Look for food particles in transport along this groove. A little carmine suspended in seawater might help visualize the role of the groove.
If the lophophore is extended, observe it and note the arrangement of tentacles around the mouth. The red color is due to hemoglobin in erythrocytes in the blood vascular system.
Look dorsal to the epistome and mouth for the anus, which is located outside the lophophore and just dorsal to it, between the arms of the horseshoe (Fig 25-3B). The anus is located on the midline and is flanked by the openings of the two metanephridia, the nephridiopores. You probably will not see the nephridiopores.
The two small concavities immediately dorsal to the epistome, but outside the lophophore, are the lophophore organs (= spermatophoral glands) (Fig 25-3B). Sperm exiting the coelom through the nephridiopores are organized into spermatophores by the lophophoral organs. The spermatophores will be transported by water currents to other phoronids. In the lophophore of the partner the sperm become amoeboid and penetrate the body wall of the metacoel to fertilize eggs present there. The resulting zygotes exit the metacoel through the nephridiopores. Several species of Phoronis brood their embryos (on the lophophore) but P. architecta is not one of them. It spawns freely into the sea. The hermaphrodite, P. hippocrepia, is a brooding species (Zimmer, pers. com).
The long trunk, or metasome, of the phoronid accounts for most of the body and contains the spacious metacoel, or trunk coelom (Fig 25-2B). The posterior end of the trunk is slightly swollen to form an ampulla.
Look inside the trunk for the descending ventral blood vessel and the ascending dorsal blood vessel (Fig 25-2B, 25-5). The red color of their corpuscular hemoglobin makes them easy to see in living specimens. They are connected to circular blood vessels at the base of the lophophore. Tentacular vessels extend from the circular vessels into each tentacle of the lophophore. At the posterior end of the trunk the dorsal and ventral vessels are connected to a plexus, or "capillary" bed in the wall of the gut and gonads.
The gut is a long "U" with one limb descending from mouth down into the trunk to the ampulla and the other limb ascending back to the anus (Fig 25-2B, 25-5). A long glandular esophagus extends from the mouth down the trunk to the expanded stomach located in the posterior end just before the gut reverses its direction. The intestine exits the stomach at the bottom of the “U” and is the ascending limb of the gut tube. It ends at the anus.
Phoronids may be either gonochoric or hermaphroditic but P. architecta is gonochoric. The gonads are located in the metacoel on the posterior end of the esophagus. Gametes are released from the gonads into the metacoel where they mature and are stored. Eventually they are fertilized and released through the metanephridia and nephridiopores. The trunk coelom may contain eggs or zygotes.
>1b. If the metacoel appears to contain gametes, open it to release them into a small dish of clean seawater. Make a wetmount of the gametes and cover it with a Vaseline-bordered coverslip (see Techniques chapter) to prevent desiccation. Examine the preparation with the compound microscope. Determine if the gametes are eggs or sperm. Eggs are large, immobile, and yolky. Sperm are tiny, flagellated, and mobile. If they appear to be "eggs" they could actually be zygotes or early embryos. Monitor the development of the embryos in the wetmount for as long as they live and then turn your attention to the embryos in the dish. If you are fortunate, they may develop into actinotroch larvae (see below) (Fig 25-6A). <
>1c. Make a wet mount of the lophophore or a few tentacles from it. Look for the ciliated epithelium and the mesocoel. Try to distinguish between frontal and lateral cilia. <
Actinotroch Larva
The planktotrophic actinotroch is the characteristic phoronid larva (Fig 25-6A). The large preoral hood at the anterior end overhangs the mouth. The anus is situated terminally at the posterior end and is surrounded by a ciliary ring, the telotroch. Just posterior to the mouth and oral hood is a girdle of ciliated larval tentacles that make the actinotroch distinctive and easily recognized. At metamorphosis, the larval tentacles may transform into adult tentacles or they may be replaced by a new set of adult tentacles, depending on taxon. Midway along the ventral surface of the larva there is an invagination of the body wall called the metasomal sac.
The actinotroch is very unwormlike and has a more or less straight, certainly not U-shaped, gut. Metamorphosis from actinotroch to worm is rapid, requiring only about 30 minutes. During metamorphosis the larva must become wormlike and must develop a much longer gut with a 180 ° bend in it. During this process, the major axis of the larva, which is originally the antero-posterior axis, becomes the dorso-ventral axis. The animal shortens along the antero-postero axis and lengthens along its dorso-ventral axis.
This occurs as the metasomal sac evaginates from the ventral surface and grows into a long wormlike body (Fig 25-7). A loop of the gut is pulled down into the everting metasomal sac and the lengthening body. Eventually, this becomes the largest part of the animal and contains the ascending and descending limbs of the gut. The old anterior end of the animal bears the lophophore and anus. In reality, what is usually called the anterior end of the animal is the anterior and posterior ends of the larva whereas the so-called posterior end of the adult is really the ventral end of the larva.
References
Emig C. 1971. Taxonomie et systematique des Phoronidiens. Bulletin. Museum National d'Histoire Naturelle 3(8), zoologie 8:473-546.
Emig C, Mittelwihr C de . 1999. What is a phoronid?
www.com.univ-mrs.fr/DIMAR/Phoro/
Herrmann K. 1997. Phoronida, pp 207-236 in Harrison FW, Woollacott RM (eds). Microscopis anatomy of invertebrates vol 13, Lophophorates, Entoprocta, and Cycliophora. Wiley-Liss, New York. 500 pp.
Hyman LH. 1959. The invertebrates, V. Smaller coelomate groups. McGraw-Hill, New York.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Zimmer R. 2001. Personal communication.
Supplies
Dissecting microscope
Small dissecting pan
Living phoronid
Isotonic magnesium chloride
Carmine/seawater suspension