Hiding by day in burrows in sand or mud bottoms, the worms come out at night to search for food, which is grabbed with powerful chitinous jaws on a muscular eversible pharynx. Biramous lateral appendages called parapodia are easily seen on this cross section. These highly vascularized appendages serve both for locomotion swimming or creeping along the bottom and respiration, and they may also be supplied with various sensory structures.
This slide shows a stained cross section through the body of a common earthworm Lumbricus terrestris. On the outside of the worm is a thin, non-living cuticle that is secreted by the underlying hypodermis. Beneath the hypodermis is a thin layer of circular muscles and a much thicker layer of longitudinal muscles. Observe the large body cavity coelom lined by a thin layer of flattened cells that make up the peritoneum. Much of the coelom is taken up by the intestine, which contains a conspicuous fold of tissue called the typhlosole.
It's thought that this structure serves to increase the surface area of the intestine for absorption. Covering the outside of the intestine and most of the inside of the typhlosole is a specialized tissue made up of chloragogue cells.
These cells are involved in a variety of metabolic functions including the synthesis of urea, glycogen, fats and hemoglobin. This slide shows a stained whole mount of a common freshwater leech Helobdella stagnalis with its proboscis extended.
Although they are known for their blood sucking habits, many leeches are actually predators that feed on soft-bodied invertebrates. During feeding, the proboscis is extended out of the mouth where it is used to penetrate the surface of the prey and suck out its body fluids. In the region of the neck a dark, circular structure can be seen. The function of this structure which is unique to this species is unknown! This slide shows a parapodium from a polychaete worm.
This biramous appendage consists of a ventral division called the neuropodium and a dorsal division called the notopodium, each of which is supported by a stiff chitinous rod called an aciculum. A dorsal cirrus and ventral cirrus richly supplied with sensory receptors project from the notopodium and the neuropodium respectively.
Numerous dark staining setae that extend beyond the parapodium can also be seen on this slide. This slide shows a another cross section through the body of a common earthworm Lumbricus terrestris.
Note the eight chitinous setae pointed to by blue arrows that are found in each segment of the earthworm except the first and last. These structures are used to help anchor the worm when burrowing and moving through the soil.
Although each seta in the living worm does project through the body wall, this section does not reveal this fact. This slide shows a cross section of the common earthworm Lumbricus terrestris. Note the coiled tubular portions of the nephridia found on each side of the coelom. These excretory organs open through nephridiopores on the ventral surface of the body wall. This specimen has been sectioned in such a way that a portion of one nephridiopore can be seen on the lower right hand side of the slide.
This image shows the anterior region of a preserved earthworm. Note the five pairs of aortic arches which have been injected with red latex that serve as "hearts" to pump blood from the dorsal blood vessel to the ventral blood vessel.
Digestive structures visible on the dissection mount include the pharynx, esophagus, thin-walled crop, muscular gizzard which serves to grind up ingested food and the beginning of the intestine. Also seen on the image are the seminal receptacles which receive sperm from the mating partner and seminal vesicles which store sperm for release to the mating partner. This image shows the middle region of a preserved earthworm.
Note the dorsal blood vessel which has been injected with red latex and ventral nerve cord that carries information from a pair of cerebral ganglia that serve as a "brain" to all segments of the worm. The large, muscular intestine is also visible on the display. This image and the three close-up views linked above is taken from a plastic model of an earthworm.
Earthworms burrow through soil, feeding on decaying organic matter. As pointed out by Charles Darwin, they perform a beneficial role by aerating the soil and enriching it by bringing up nutrients from below. In terms of nutrition, food is brought into mouth by a muscular pharynx that passes through the esophagus, which is surrounded by calciferous glands that remove excess calcium ions acquired from eating the soil from the blood and secrete them into the gut.
Food then enters a thin-walled crop and is passed on to a muscular gizzard, which serves as a grinding organ. From there it passes into the intestine to exit out the terminal anus. The dorsal wall of the intestine is folded inward to form a typhlosole that serves to increase the surface area for digestion and absorption. Chloragogue cells surrounding the intestine and filling much of the typhlosole are involved in the synthesis of glycogen and fats.
Although much of the circulation in annelids is handled by the coelom, earthworms also have a well-developed, closed circulatory system consisting of a dorsal vessel that runs above the alimentary canal from the anus to the pharynx.
The dorsal vessel receives blood from the body wall and pumps it anteriorly into five pairs of aortic arches that help maintain a steady pressure into the ventral vessel, which delivers blood to the rest of the body. In terms of excretion, some wastes simply diffuse out through the moist skin, which also serves as the principal gas exchange organ. Other wastes are handled by paired structures called nephridia.
Each nephridium also called a metanephridium has a ciliated funnel-like nephrostome that collects wastes from the coelomic fluid and then passes it through the transverse septum into the next metamere. The nephridia empty to the outside via a openings called nephridiopores. In terms of reproduction, although earthworms are monoecious, they practice cross fertilization. Sperm are released from the seminal vesicles of one partner and received by seminal receptacles of the other after passing along a seminal groove.
After copulation, the clitellum of each worm secretes a cocoon that receives the sperm and eggs, which are then fertilized in the cocoon. The cocoon is then deposited in the ground, where direct development takes place, terminating when a young earthworm that resembles the adult hatches from the cocoon.
Prostomium; 2. Mouth; 3. Buccal cavity; 4. Esophagus; 5. Aortic arch; 6. Nephridia 7. Cerebral ganglion; 8. Circumpharyngeal connective; 9. Ventral nerve cord. Pharynx; 2. Esophagus; 3. Crop; 4. Aortic arches; 5. Dorsal blood vessel; 6. Ventral blood vessel; 7. Nephridia a pair in each segment ; 8. Seminal vesicles. Esophagus 2.
Crop; 3. Gizzard; 4. Chloragogue cells; 5. Intestine; 6. Dorsal blood vessel; 7. Ventral blood vessel; 8. Setae; 9. Ventral nerve cord; This dissection of a preserved earthworm shows some of the more conspicuous features of its internal anatomy. Digestive structures that can be seen include the buccal cavity 1 , pharynx 2 , crop 3 and gizzard 4. Also seen on the image are the seminal vesicles 6 , seminal receptacles 7 , the dorsal blood vessel 5 that runs along the top of the intestine, two of the many septa 8 that divide the coelom into separate segments called metameres and one of the five pairs of aortic arches 9 that help pump blood from the dorsal blood vessel to the ventral blood vessel.
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. 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. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates. The subclass Brachiobdella includes species like Branchiobdella balcanica sketi and Branchiobdella astaci , worms that show similarity with leeches as well as oligochaetes.
Figure 3. The a earthworm, b leech, and c featherduster are all annelids. Phylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism.
Annelids are protostomes. These animals have well-developed neuronal and digestive systems. Some species bear a specialized band of segments known as a clitellum. Annelids show the presence numerous chitinous projections termed chaetae, and polychaetes possess parapodia. Suckers are seen in order Hirudinea. Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism. Internal segmentation is absent in class Hirudinea. Improve this page Learn More. Skip to main content.
Module Invertebrates. Search for:. This is a combination video and animation on annelid anatomy. Try It.
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