The ventral surface of worms and other organisms is the bottom side of the body, often closest to the ground. The dorsal surface is located on the upper part of the body facing the sky. The lateral surfaces are found on the left and right sides of the body. Figure 3. Organs for sensing light, touch, and smell are concentrated in the heads of worms. They can detect the kinds of environment they encounter by moving in the anterior direction.
There are six features and systems that reveal an evolving complexity in the body structure of most worms:. The phylum Platyhelminthes consists of simple worm-like animals called flatworms Fig. Flatworms live on land, in fresh water, in the ocean, and in or on other animals as parasites e. Parasitic flatworms that live on or inside other animals—including humans—can injure or even kill the host organism. Free-living non-parasitic flatworms are typically less than 10 centimeters long. Marine species live buried in the sand or under rocks in shallow water.
All free-living flatworms are predators that actively hunt for food. Some live symbiotically with crabs, clams, oysters, shrimp, and barnacles. Some marine flatworms are brilliantly colored Fig. Flatworms are more complex than cnidarians. Cnidarians have two layers of cells, the ectoderm and the endoderm; flatworms have a middle layer called the mesoderm between the other two layers Fig. This extra layer is important because its cells specialize into a muscular system that enables an animal to move around.
Beginning with the flatworms, all the animals we will subsequently study have a mesoderm and muscular system. The cells of the ectoderm and endoderm are also more organized than similar cells of cnidarians.
For the first time, we see groups of tissues that have evolved to form organs, such as the ones in the digestive, nervous, and excretory systems. Like the cnidarians, flatworms have a digestive system with only a single opening into the digestive cavity, but in independently living marine flatworms the cavity branches into all parts of the body Fig. These flatworms feed through a pharynx.
A pharynx is a long, tubular mouthpart that extends from the body, surrounds the food, and tears it into very fine pieces Fig. Cells lining the digestive cavity finish digesting the food. Then the dissolved nutrients move to other cells of the body.
Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms usually absorb their nutrients directly from the host, while parasitic flukes have retained a digestive system.
Like most self-propelling animals, independent-living flatworms have a central nervous system. A central nervous system consists of a mass of nerve cells, called a ganglion , in more complex organisms, the ganglion evolves into a brain in the anterior part of the body, and a nerve cord extending from the brain toward the posterior end of the body Fig. Sensory cells in the head detect changes in the environment.
In free-living flatworms, sensory cells that respond to light are clustered in two eyespots in the head. Sensory cells that detect water currents, solid objects, and chemicals are in two flap-like projections on the head called auricles. In self-propelling animals, these sensory organs in the head are the first part of the animal that encounters new surroundings.
The ganglion receives information from the sensory structures and sends signals to other parts of the body along two strands of nerve cells running toward the tail. The excretory system removes waste products and excess water from tissues of flatworms. 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. Ab aap Whatsapp pe solutions paa saktey h, hum aapko message karenge. Ab aap Whatsapp pe solutions paa saktey h, hum aapko ping karenge. Study Materials. Why use Doubtnut? Instant Video Solutions. Request OTP. Animal Morphology Earthworms belong to which phy Updated On: Share This Video Whatsapp.
Her poems have appeared in "Small Craft Warnings. How Do Earthworms Move? How Do Earthworms Protect Themselves? The Anatomy of the Hydra. Worms in the Ecosystem. Cephalization of Earthworms. What Does a Roundworm Look Like? How Do Roundworms Reproduce?
How Do Worms Move? Animals That Breathe Through the Skin.
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