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.
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. 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. 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.
0コメント