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. Some polychaete worm eyes can even detect shapes. Nerves also extend from the brain around the digestive tube and along the ventral surface. A ganglion or cluster of nerve cells operates the organs in each segment. The excretory system of annelid worms consists of a pair of small tubes in each segment.
These tubes, called nephridia from the Greek root word nephrus meaning kidney , are open at both ends. They filter coelomic fluid, which contains useful nutrient molecules along with waste molecules. As the fluid moves through the tube, useful molecules return to the coelom, and waste molecules pass into the water. This document may be freely reproduced and distributed for non-profit educational purposes.
Skip to main content. Search form Search. Join The Community Request new password. Main menu About this Site Table of Contents. Worms: Phyla Platyhelmintes, Nematoda, and Annelida.
MS-LS Use argument based on empirical evidence and scientific reasoning to support an explanation for how characteristic animal behaviors and specialized plant structures affect the probability of successful reproduction of animals and plants respectively.
MS-LS Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms. MS-LS Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.
MS-LS Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.
HS-LS Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. HS-LS Communicate scientific information that common ancestry and biological evolution are supported by multiple lines of empirical evidence. The content and activities in this topic will work towards building an understanding of the worms in the phyla Platyhelminthes, Nematoda, and Annelida.
Introduction to Worms Most people are familiar with earthworms found in garden soil. Roundworms: Phylum Nematoda Species in the phylum Nematoda from the Greek root word nema meaning thread are better known as the roundworms Fig. Segmented Worms: Phylum Annelida The worms in the phylum Annelida from the Latin root word annelus meaning ring typically have complex segmented bodies Fig.
These features appear in some form in all larger, more complex animals: a coelom , a body cavity between the digestive tube and the external body wall that is lined with tissue a circulatory system consisting of a series of tubes vessels filled with fluid blood to transport dissolved nutrients, oxygen, and waste products rapidly and efficiently Recall that the coelom is a fluid-filled cavity lying between the digestive tube and the outer body tube and surrounded by mesodermal tissue. Further Investigations.
Special Features:. Representative Image:. Further Investigations: What is an Invertebrate? Question Set: What is a Mammal? Further Investigations: What is a Mammal? Share and Connect. We invite you to share your thoughts, ask for help or read what other educators have to say by joining our community. However, the offspring do not fully detach from the parents and therefore resemble a chain in appearance. All of the remaining flatworms discussed here are part of the Rhabditophora.
Many flatworms are parasitic, including important parasites of humans. Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues from ectoderm , internal tissues from mesoderm , and line the digestive system from endoderm.
The epidermal tissue is a single layer cells or a layer of fused cells syncytium that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface.
The flatworms are acoelomates, so their bodies are solid between the outer surface and the cavity of the digestive system. The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts. Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched.
Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body.
The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells. Figure 1. The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus.
The excretory system is made up of tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.
There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. Most flatworm species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups.
Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda Figure 2. View this video to see nematodes move about and feed on bacteria. Nematodes employ a diversity of sexual reproductive strategies depending on the species; they may be monoecious, dioecious separate sexes , or may reproduce asexually by parthenogenesis. Caenorhabditis elegans is nearly unique among animals in having both self-fertilizing hermaphrodites and a male sex that can mate with the hermaphrodite.
Arthropoda dominate the animal kingdom with an estimated 85 percent of known species, with many still undiscovered or undescribed. The principal characteristics of all the animals in this phylum are functional segmentation of the body and the presence of jointed appendages [Figure 4]. As members of Ecdysozoa, arthropods also have an exoskeleton made principally of chitin. Arthropoda is the largest phylum in the animal world in terms of numbers of species, and insects form the single largest group within this phylum.
Arthropods are true coelomate animals and exhibit prostostomic development. Figure 4: Trilobites, like the one in this fossil, are an extinct group of arthropods. A unique feature of arthropods is the presence of a segmented body with fusion of certain sets of segments to give rise to functional segments. Fused segments may form a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk. The coelom takes the form of a hemocoel or blood cavity. The open circulatory system, in which blood bathes the internal organs rather than circulating in vessels, is regulated by a two-chambered heart.
Respiratory systems vary, depending on the group of arthropod: Insects and myriapods use a series of tubes tracheae that branch throughout the body, open to the outside through openings called spiracles , and perform gas exchange directly between the cells and air in the tracheae.
The book gills of crustaceans are external structures similar to book lungs with stacks of leaf-like structures that exchange gases with the surrounding water [Figure 5]. Figure 5: The book lungs of a arachnids are made up of alternating air pockets and hemocoel tissue shaped like a stack of books.
The book gills of b crustaceans are similar to book lungs but are external so that gas exchange can occur with the surrounding water. Arthropod Diversity Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats.
The phylum is further classified into five subphyla: Trilobitomorpha trilobites , Hexapoda insects and relatives , Myriapoda millipedes, centipedes, and relatives , Crustacea crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton , and Chelicerata horseshoe crabs, arachnids, scorpions, and daddy longlegs. Trilobites are an extinct group of arthropods found from the Cambrian period — million years ago until they became extinct in the Permian — million years ago that are probably most closely related to the Chelicerata.
The 17, described species have been identified from fossils [Figure 4]. The Hexapoda have six legs three pairs as their name suggests. Hexapod segments are fused into a head, thorax, and abdomen [Figure 6]. The thorax bears the wings and three pairs of legs. The insects we encounter on a daily basis—such as ants, cockroaches, butterflies, and bees—are examples of Hexapoda.
Figure 6: In this basic anatomy of a hexapod, note that insects have a developed digestive system yellow , a respiratory system blue , a circulatory system red , and a nervous system purple. Subphylum Myriapoda includes arthropods with legs that may vary in number from 10 to This subphylum includes 13, species; the most commonly found examples are millipedes and centipedes.
All myriapods are terrestrial animals and prefer a humid environment [Figure 7]. Figure 7: a The centipede Scutigera coleoptrata has up to 15 pairs of legs. Crustaceans, such as shrimp, lobsters, crabs, and crayfish, are the dominant aquatic arthropods. A few crustaceans are terrestrial species like the pill bugs or sow bugs. The number of described crustacean species stands at about 47, Although the basic body plan in crustaceans is similar to the Hexapoda—head, thorax, and abdomen—the head and thorax may be fused in some species to form a cephalothorax , which is covered by a plate called the carapace [Figure 8].
The exoskeleton of many species is also infused with calcium carbonate, which makes it even stronger than in other arthropods. Crustaceans have an open circulatory system in which blood is pumped into the hemocoel by the dorsal heart. Most crustaceans typically have separate sexes, but some, like barnacles, may be hermaphroditic. Serial hermaphroditism, in which the gonad can switch from producing sperm to ova, is also found in some crustacean species.
Larval stages are seen in the early development of many crustaceans. Most crustaceans are carnivorous, but detritivores and filter feeders are also common. Figure 8: The crayfish is an example of a crustacean. It has a carapace around the cephalothorax and the heart in the dorsal thorax area.
Subphylum Chelicerata includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders. This subphylum is predominantly terrestrial, although some marine species also exist. An estimated , 2 described species are included in subphylum Chelicerata.
The phylum derives its name from the first pair of appendages: the chelicerae [Figure 9] a , which are specialized mouthparts.
The chelicerae are mostly used for feeding, but in spiders, they are typically modified to inject venom into their prey [Figure 9] b. As in other members of Arthropoda, chelicerates also utilize an open circulatory system, with a tube-like heart that pumps blood into the large hemocoel that bathes the internal organs. Aquatic chelicerates utilize gill respiration, whereas terrestrial species use either tracheae or book lungs for gaseous exchange.
Figure 9: a The chelicerae first set of appendages are well developed in the Chelicerata, which includes scorpions a and spiders b. Click through this lesson on arthropods to explore interactive habitat maps and more.
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