Flatworms reflect several major evolutionary advances in invertebrates. They have three embryonic cell layers, including mesoderm. The mesoderm layer allows them to develop organ systems. For example, they have muscular and excretory systems.
The muscular system allows them to move from place to place over solid surfaces. The excretory system lets them maintain a proper balance of water and salts. Flatworms also show cephalization and bilateral symmetry. Flatworms reproduce sexually. In most species, the same individuals produce both eggs and sperm. There may be several different larval stages. The final larval stage develops into the adult form, and the life cycle repeats.
Both flukes and tapeworms are parasites with vertebrate hosts, including human hosts. Look at the life cycle of the liver fluke in Figure below. As hunters today, flatworms hunt prey with their head leading the way. Flatworms are hermaphrodites: an individual flatworm is both male and female. When flatworms mate, the worm that first receives sperm, carries the fertilized eggs.
They were the first animals with internal fertilization. The body plan of the flatworm is bilateral, with a head that leads—a plan that continues in most animals living today. The structure of this bilateral plan is ideal for an active hunter. They hunt by waiting for its prey to open its valves to feed.
The life cycle of the flatworm is not well known. Flatworms Order: Polycladida. Quick Facts Species Native Size Smaller than a quarter Habitat Lives among reefs, piers, jetties and other hard surfaces in shallow waters.
Range Abundant throughout the middle and lower Chesapeake Bay. 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.
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