Muscles surrounding the tube contract, squeezing the food and pushing it along in a process called peristalsis. Indigestible wastes pass out through the anus. Unlike flatworms, nematodes are slender, and they are covered by a protective cuticle. A cuticle is a waxy covering secreted by the epidermis, or outermost cellular tissue. Because of this covering, gas exchange cannot occur directly across the skin as in flatworms. Rather, gas exchange and waste excretion in nematodes occurs by diffusion across the wall of the gut.
Although nematodes do have a space in the body between the digestive tract and the body wall, it is not lined with tissue and is not considered to be a true coelom. Thus, nematodes are sometimes referred to as pseudocoelomates Fig. Most worms have two bands of muscles: longitudinal muscles that run the length of the body and circular muscles that form circular bands around the body.
Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles. This explains their characteristic thrashing movement, as they can move only by contracting the long muscles on either side of their body and wriggling forward. The nervous system of nematodes consists of a set of nerves that run the length of the body and connect to anterior ganglia.
Free-living nematodes are capable of sensing light with ocelli, and most nematodes have fairly complex chemosensory abilities. Most nematodes are not hermaphrodites , with both sexes in one individual, but are known as dioecious —having individuals of separate sexes. Their chemosensory abilities are very helpful, as they rely on pheromones to locate potential mates. The worms in the phylum Annelida from the Latin root word annelus meaning ring typically have complex segmented bodies Fig.
The body of an annelid is divided into repeating sections called segments with many internal organs repeated in each segment. Earthworms class Oligochaeta are familiar terrestrial members of this phylum and leeches class Hirudinea are well-known parasitic members of the phylum, most commonly found in freshwater. They occur mostly in marine and brackish water habitats. Polychaete from the Greek root words poly meaning many and chaeta meaning bristle annelid worms are so named because most of their segments have bristles called chatae or setae.
The free-moving not sessile polychaetes have muscular flaps called parapodia from the Greek para meaning near and podia meaning feet on their sides, and the setae on these parapodia dig into the sand for locomotion. Fireworms are a type of polychaete that have earned their name from stinging bristles on each parapodium Fig. These bristles can penetrate human skin, causing irritation, pain and swelling, similar to the irritation caused by exposure to fiberglass. Tubeworms are sessile polychaetes that live in tubes that they build by secreting the tube material.
The tubes, attached to rocks or embedded in sand or mud, may be leathery, calcareous, or sand-covered depending on the worm species Fig. Tubeworms feed by extending tentacles from the tube. Bits of food move along grooves in the tentacles to the mouth. Some tubeworms retract their tentacles when food lands on them. Tubeworms use their parapodia to create currents of water that flow through the tubes to aid in respiration and help clean the tubes. By contrast, the free-living or mobile polychaete worms have a proboscis that can extend from their mouths to catch prey.
This is a feeding organ that is often armed with small teeth or jaws on its tip. With their active lifestyle and good defenses, free-moving polychaetes can make their living in a variety of habitats such as mud, sand, sponges, live corals, and algae. Like flatworms, annelids have a mesoderm with muscle, a central nervous system, and an excretory system. Each of these systems is more complex in the annelid than in flatworms or nematodes.
In addition to a more specialized complete digestive system, annelid worms have also evolved body features not found in flatworms or nematodes. These features appear in some form in all larger, more complex animals:. Recall that the coelom is a fluid-filled cavity lying between the digestive tube and the outer body tube and surrounded by mesodermal tissue. The digestive tube lies inside the outer body tube.
The fluid in the coelom supports the soft tissues of the body wall much as it does in the hydrostatic skeleton of cnidarians. Mesodermal muscles in the wall of the body tube and digestive tube can put pressure on the fluid to aid in movement.
In the body wall of the annelids are two types of muscles: circular and longitudinal. When the circular muscles contract, the segment gets longer and narrower. When the longitudinal muscles contract, the segment gets shorter and fatter Fig. These contractions produce the crawling movement of worms.
Recall that nematodes lack circular muscles, and can only move by contracting their longitudinal muscles, thus thrashing and wriggling rather than crawling. The setae along the body of polychaetes stick in the substrate, holding parts of the worm in place while other parts move forward. Annelids have a closed circulatory system in which blood is pumped along by muscles in blood vessels Fig.
Blood flows through the microscopic capillaries, picking up food molecules from the digestive tract and oxygen from the skin and transporting them to the cells of the body. The parapodia, the flaps on the sides of the segments, increase the surface area of the skin for respiration. Such a system lets animals grow much larger than possible in the flatworms, which must rely on diffusion.
The nervous system is also more complex in annelids than in other worm-like phyla. Annelids have a simple brain organ consisting of a pair of nerve clusters in the head region Fig. Nerves link the brain to sensory organs in the head that detect the environment in front of the worm.
Earthworms are eyeless, but polychaete annelids have eyes that can distinguish between light and dark. If we accept that the basic topology shown in Figure 1 is correct, but do not root the tree, then a diagram as shown in Figure 5 is the result. This may represent the most conservative representation of our understanding of annelid relationships. Aguinaldo, A. Turbeville, L. Linford, M. Rivera, J. Garey, R. Raff, and J. Evidence for a clade of nematodes, arthropods and other moulting animals.
Nature Bartolomaeus, T. Structure and formation of the uncini in Pectinaria koreni, Pectinaria auricoma Terebellida and Spirorbis spirorbis Sabellida : implications for annelid phylogeny and the position of the Pogonophora. Brown, S. Rouse, P. Hutchings, and D. Cuvier, G. Deterville, Paris. Eibye-Jacobsen, D. A new genus and species of Dorvilleidae Annelida, Polychaeta from Bermuda, with a phylogenetic analysis of Dorvilleidae, Iphitimidae and Dinophilidae.
Fauchald, K. The polychaete worms. Definitions and keys to the orders, families and genera. Los Angeles County. Polychaete systematics: Past and present. Hessling, R. Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis. Jamieson, B. The ultrastructure of the Oligochaeta.
Academic Press, London. Lamarck, J. Ultrastructure of larval and adult setae of Brachiopoda. Zoologischer Anzeiger The origin of the coelom in Brachiopoda and its phylogenetic significance. The phylogenetic position of Brachiopoda- a comparison of morphological and molecular data.
Martin, P. On the origin of the Hirudinea and the demise of the Oligochaeta. Royal Soc. London Ser. B: Biol. McHugh, D. Molecular evidence that echiurans and pogonophorans are derived annelids. Newby, W. The embryology of the echiuroid worm Urechis caupo. Purschke, G. Hessling, and W. The phylogenetic position of the Clitellata and the Echiura - on the problematic assessment of absent characters.
Rouse, G. A cladistic analysis of Siboglinidae Caullery, Polychaeta, Annelida : formerly the phyla Pogonophora and Vestimentifera. The articulation of annelids.
Cladistics and polychaetes. Sedgwick, A. A student's textbook of zoology. Siddall, M. Apakupakul, E. Burreson, K. Coates, C. Erseus, S. Gelder, M. Kallersjo, and H. Validating Livanow: Molecular data agree that leeches, branchiobdellidans, and Acanthobdella peledina form a monophyletic group of oligochaetes. Stechmann, A. Analysis of the complete mitochondrial DNA sequence of the brachiopod Terebratulina retusa places Brachiopoda within the protostomes.
Storch, V. Zur vergleichenden Anatomie der segmentalen Muskelsysteme und zur Verwandtschaft der Polychaeten-Familien. Tiere Westheide, W. The direction of evolution within the Polychaeta. McHugh, G. Some live in water, and some live on land. Burrowing annelids, like the earthworm, play an important role in helping organic matter decompose. Earthworms eat dead plants and animals. When they eat, they also take in soil and tiny pebbles. Earthworms take in nutrients from microorganisms in the material they ingest.
They then excrete wastes in the form of casts. Casts are rich in nutrients like nitrogen, phosphorus, and potash. In addition to breaking down organic materials and adding nutrients to the soil, earthworms also help loosen the soil so air can circulate.
0コメント