Why platyhelminthes are flat

As an adult, the fluke has a vertebrate host. As a larva, it has an invertebrate host. If you follow the life cycle, you can see how each host becomes infected so the fluke can continue its life cycle. Life Cycle of the Sheep Liver Fluke. The sheep liver fluke has a complicated life cycle with two hosts. How could such a complicated way of life evolve? Tapeworms and flukes have suckers and other structures for feeding on a host.

Tapeworms also have a scolex , a ring of hooks on their head to attach themselves to the host see Figure below. Unlike other invertebrates, tapeworms lack a mouth and digestive system. Tapeworm Suckers and Hooks.

The head of a tapeworm has several suckers. Not all flatworms are parasites. Some are free-living carnivores. 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. Parasitic flatworms protect themselves from the hosts' digestive liquids by developing teguments or coverings around their bodies.

Tapeworm , also called cestode, any member of the invertebrate class Cestoda phylum Platyhelminthes , a group of parasitic flatworms containing about 5, species. The disease caused by tapeworms is known as cestodiasis q. Flatworms have a number of natural predators, including the Sixline Wrasse Pseudocheilinus hexataenia , the Yellow Wrasse, and the Spotted Mandarin. The biggest drawback to ridding your tank of flatworms with this method is that the fish will not consume every flatworm in the tank.

Any of the worms e. It should now come as no surprise to you that all of the animals in the Platyhelminthes phylum are flatworms! There are three main types of flatworms : flukes, planarians, and tapeworms.

Platyhelminthes Gr. Platy- flat; helminthes- worm They are advanced diploblastic or lower triploblastic , acoelomate and bilaterally symmetrical metazoan. Why are platyhelminthes flat? Category: movies science fiction movies.

Flatworms have a flat body because they lack a fluid-filled body cavity. They also have an incomplete digestive system with a single opening. The mesoderm layer allows flatworms to develop muscle tissues so they can move easily over solid surfaces. Flatworms have a concentration of nerve tissue in the head end.

Are flatworms dangerous? Are flatworms poisonous? 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.

As discussed above, the relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group, a group that does not have a single common ancestor. Figure 2. Phylum Platyhelminthes is divided into four classes. Dactylogyrus , commonly called a gill fluke, is about 0. The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments.

The ventral epidermis of turbellarians is ciliated and facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment. The monogeneans are ectoparasites, mostly of fish, with simple lifecycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form.

The parasite has only one host and that host is usually only one species. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles.

Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common. The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs.