How many tissue layers do flatworms have

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.

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. Flatworms have a surprisingly elaborate system to rid the body of wastes Fig. This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body.

Connected to the tubes are tiny cells that move wastes and water from the tissues into the tubes. These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores. Under a microscope the flagellar movement looks like a flickering fire, and the structure is called a flame bulb.

Flatworms have no circulatory system. Animals without a circulatory system have limited abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave. As molecules spread through water, they become less concentrated as they move away from their source. This is known as diffusion. But cnidarians have no problem with diffusion because most cells of their bag-shaped bodies are in direct contact with the water, making the exchange of oxygen and nutrients easy Fig.

Flatworms, bag-shaped but flattened, also get oxygen and nutrients to their body cells easily because all their cells are close to either their outer surface or their digestive cavity Fig.

As animals become larger and more complex, diffusion is often no longer an option, and then we begin to see the development of circulatory and respiratory systems.

Species in the phylum Nematoda from the Greek root word nema meaning thread are better known as the roundworms Fig. There are about 25, species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth.

One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10, nematodes in cubic centimeters cm 3 of soil.

Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails. Like flatworms, roundworm species adopt either a free-living or a parasitic lifestyle. Parasitic nematodes Fig. Many nematodes that are parasitic on plants can devastate crops. Some nematodes are cryptobiotic and have demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable.

Like the flatworms, nematodes are bilaterally symmetrical. They take their name from their round body cross-sectional shape. Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system.

An animal with a complete digestive system has a mouth at one end, a long tube with specialized parts in the middle, and an anus at the other end. With a complete digestive system an animal can eat while its previous meal digests. Parts of the digestive system can specialize to do different jobs, digesting food in stages Fig. As the food moves along, it is broken into molecules and absorbed by the cells lining the tube.

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.

The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke Schistosoma. The disease infects an estimated million people in the tropics and leads to organ damage and chronic symptoms including fatigue.

Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body.

The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but will contain reproductive structures, both male and female. Most reproduction occurs by cross-fertilization. The proglottid detaches and is released in the feces of the host.

The fertilized eggs are eaten by an intermediate host. The juvenile worms emerge and infect the intermediate host, taking up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are acquired by eating uncooked or poorly cooked pork, beef, and fish. The phylum Nematoda , or roundworms, includes more than 28, species with an estimated 16, parasitic species.

Most nematodes look similar to each other: slender tubes, tapered at each end Figure Nematodes are pseudocoelomates and have a complete digestive system with a distinct mouth and anus. The nematode body is encased in a cuticle, a flexible but tough exoskeleton, or external skeleton, which offers protection and support.

The cuticle contains a carbohydrate-protein polymer called chitin. The cuticle also lines the pharynx and rectum. Although the exoskeleton provides protection, it restricts growth, and therefore must be continually shed and replaced as the animal increases in size.

There may also be a sharp stylet that can protrude from the mouth to stab prey or pierce plant or animal cells. The mouth leads to a muscular pharynx and intestine, leading to the rectum and anal opening at the posterior end. In nematodes, the excretory system is not specialized. Nitrogenous wastes are removed by diffusion. In marine nematodes, regulation of water and salt is achieved by specialized glands that remove unwanted ions while maintaining internal body fluid concentrations.

Most nematodes have four nerve cords that run along the length of the body on the top, bottom, and sides. Beneath the epidermis lies a layer of longitudinal muscles that permits only side-to-side, wave-like undulation of the body. 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 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. 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