Pharynx is the muscular tube which pumps the food into the flatworm’s gastrovascular or digestive cavity. It is located in the ventral side, which is near the mouth of the digestive cavity.

D. pharynx

Scolex refers to the head or the neck of an adult tapeworm.

B. scolex

Pseudocoelom is a body cavity that lies between the endoderm and mesoderm tissues of a roundworm.

B. pseudocoelom

The nervous system of a roundworm consists of a network of nerves called ganglia.

C. ganglia

The nephridia found in an earthworm is responsible for removing the metabolic wastes from its body.

B. nephridia

In the feeding and digestive functions of an earthworm, the food is stored in an organ called crop, which is represented by number 2 in the diagram.

B. 2

In the reproductive function of an earthworm, the segment that secretes a mucous ring wherein fertilization takes place is called a clitellum.

C. reproduction

A flexible tongue-like structure found in mollusks is called a radula. It is used as the feeding function in mollusks.

A. radula

The nephridia found in a mollusk is responsible for removing the metabolic wastes, which is in the form of ammonia, from the blood and for releasing it away from the body.

D. nephridia

Members of the class Bivalvia are mollusks that are enclosed with two shells that are hinged by a muscle. Examples of bivalves are clams, mussels, oysters, and scallops.

C. Bivalvia

Invertebrates that are lacking a coelom, or a fluid-filled cavity that lies between their body tissues, are called acoelomates. An example of an acoelomate is a flatworm. On the other hand, invertebrates that contain a coelom between the body tissues are called coelomates. An example of coelomate is a mollusk.

**Flatworms** are organisms under Phylum Platyhelminthes. They are characterized as soft, flattened organisms that have the simplest embryonic germ layers, and show bilateral symmetry and cephalization.

Because these organisms have flat and thin bodies, a lot of flatworm species do not have circulatory system to transport and move materials and nutrients in their body rather they rely on **diffusion** to transport and excrete materials in their body. Some flatworms have **flame cells** that play a role in excretion–especially in the removal of excess water in their body.

**Tubellarians** are free-living, bottom dwellers flatworms that are located usually in marine or freshwater environments. These species show a variety of colors, shapes, and sizes. One of the most known flatworms is the cross-eyed planarians.

These organisms have a single-mouth opening called the mouth, and near it is the muscular tube referred to as their *pharynx*. This pharynx extends out of the mouth to feed and then pumps or moves the food to the digestive cells or gut.

**Flatworms** are soft, flattened worms that contain three embryonic germ layers and internal organs. They also exhibit bilateral symmetry and cephalization.

These organisms have a more complex nervous system in detecting external stimuli compared to cnidarians and sponges, because these species have a series of *nerve cells* referred to as **ganglia** that act as their brain. Moreover, some species have the so-called **eyespot** that can detect the changes in amount and presence of light in the environment.

***Schistosoma*** species are under class Trematoda. They are considered parasites, infecting the internal organs of their host organisms including their blood. One known *Schistosoma* species is the blood fluke (Scientific name: *Schistosoma mansoni*).

**Schistosomiasis** is the infection caused by *Schistosoma* flukes. This infection poses danger to the host organism as the eggs of these species can cause clogging of the blood vessels, resulting in the swelling and decay of various organs such as the spleen, liver, and lungs. **Schistosomiasis** affects thousands of people all over the world, and it is most common in those tropical areas lacking proper sewage treatment facilities.

Hence, in order to address the emerging problem, it is important to have enough proper sewage treatment facilities in every region, and to contact regular water testing to the most used water sources such as drinking water facilities, and recreational water facilities/

First adaptation: Tapeworms are hermaphrodite.

They have both male and female sex organs which allows them to undergo self-fertilization that produces many zygotes which increase their chances of survival.

Second adaptation: Tapeworms can have two hosts within a life cycle.

A single tapeworm has many body segments called proglottids, wherein the youngest segments are situated at the anterior end, while the mature ones are situated at the posterior end. The mature segments break off when they release zygotes. Once the zygotes are released, it comes out of the host’s body together with the feces. If a food becomes contaminated with the zygotes and gets consumed by an animal such as a pig or a cow, it will enter its body and hatch into a larva. The infected animal becomes an intermediate host. The larvae will become dormant until the meat of the intermediate host gets consumed by a human host. Once the larva goes inside the intestines of a human, it will grow into an adult and the cycle will begin again.

First adaptation: Tapeworms are hermaphrodite.

Second adaptation: Tapeworms can have two hosts within a life cycle.

A single tapeworm has many body segments called proglottids, wherein the youngest segments are situated at the anterior end, while the mature ones are situated at the posterior end. The mature segments have both male and female sex organs which allow them to undergo self-fertilization. Once the egg cells are fertilized, the mature segments break off and release the zygotes. Once the zygotes are released, they come out of the host’s body together with the feces.

**Roundworms** are the unsegmented type of worms that have pseudocoeloms and a digestive tract containing two openings: mouth and anus. The body plan of these organisms is usually described as **tube-within-a-tube** because it has an inner tube acting as its digestive tract and an outer tube acting as its body wall.

Similar to flatworms, these species facilitate respiration, circulation, and excretion through the process of **diffusion**. Roundworms exchange oxygen and carbon dioxide and release waste materials through their body walls.

Life Cycle of a Trichinella Roundworm

1. Male and female roundworms mate and undergo internal fertilization.
2. Zygotes hatch into larvae, then the larvae are released into the bloodstream and they burrow into the tissues of the host.
3. The larvae inside the muscle tissues go into dormancy and form cysts.
4. Larvae mature once another animal eats the cysts in muscle tissues. The life cycle starts again as soon as the larvae enter another host.

***Ascaris lumbricoides*** are parasitic roundworms that infect humans and other vertebrate organisms. This worm is the culprit for the emerging malnutrition worldwide. They are very successful in invading the host’s small intestine and absorbing the nutrients that come from the food intake of the host.

*A. lumbricoides* are usually spread by eating contaminated fecal-contaminated food materials that are not washed properly. Examples of these are fruits, vegetables, and raw meat.

Since the DNA sequence of C. elegans has been identified, this discovery will allow the scientists to use this as a basis for understanding how eukaryotes evolved and became multicellular. They can use their findings of the genetic makeup of C. elegans in comparing or correlating it with other organisms’ genetic material to know the similarities and differences and to shed some light about the evolutionary history of organisms.

**Annelids** are types of worms that are known to have segments in their bodies and true coelom. There is three adaptions for feeding in annelids and they are: **Sharp jaws** that is connected to the pharynx in those carnivorous annelid species, other species have a **sticky body** for feeding on rotting vegetation. Lastly, some annelids obtain nutrition by **filter feeding**

Earthworms are an example of annelid species usually found in the soil or water environment. Earthworms moves by alternating the contraction between the circular muscles that make the worm thin and long and the longitudinal muscles that make the worm short and fat.

In reproductive biology, **hermaphrodite** is any organism that exhibits both male and female reproductive organs. These organs are also functioning well producing both male and female gametes. Example species: Angiosperms (flowering plants), and flatworms.

**Annelids** have a ringlike body segment appearance. The body of these organisms is divided into many segments that are separated from each other through a structure called septa. Each segment of their body is composed of similar structures such as eyes and other sense organs. Annelids have a true coelom, and this is covered by a mesoderm.

Annelid species can either be aquatic or land-dwelling. **Aquatic annelids** have specialized structures called *gills* that aid them in breathing and gas exchange. On the other hand, **land-dwelling annelids** has *moist skin* that enables them to respire and exchange oxygen and carbon dioxide efficiently.

Both mollusks and annelids remove their metabolic wastes using nephridia.

Their blood is pumped by a heart and it is contained by a network of blood vessels.

They are coelomates. Both annelids and mollusks have true coeloms between their body tissues.

**Mollusks** species are characterized as soft-bodied animals that usually have an internal or external shell made up of calcium carbonate. There are three major groups of mollusks namely: **Gastropods, bivalves,** and **cephalopds**. Each group has its distinct feeding mechanisms.

**Gastropods** use a flexible tongue structure, in which hundreds of teeth are attached to scrape algae off the rock and eat soft tissues of plants. While **bivalves** filter the water siphoned through their gills for food. Lastly, **cephalopods** have strong tentacles that they use to grab their prey.

An open circulatory system contains a heart that pumps blood into an open cavity. The blood diffuses into the cells and goes back into the circulation. This type of circulatory system lacks arteries and veins. Therefore, the blood surrounds the tissues and organs.

**Aquatic mollusks** such as clams, snails, and octopus, breathe and exchange gases through the use of gills located inside their mantle cavity. As the water moves through this cavity, oxygen molecules are transported into the blood through these gills. At the same time, carbon dioxide waste moves out from the blood into the water.

On the other hand, **Aquatic annelids** have specialized structures called gills that aid them in breathing and gas exchange. Some annelid species like feather-duster worms have their gills as feathery structures protruding outside their body tube.

**Mollusks** are members of the phylum Mollusca. These species are characterized by having a soft body, which usually has an internal or external shell made up of calcium carbonate, secreted by their body.

Mollusks reproduce in many ways. Most gastropods and bivalves reproduce sexually through **external fertilization**. These species release **a lot of** eggs and sperms into open water, then fertilization also happens in the water. The fertilized eggs will then develop into free-swimming larvae, then mature to be adult gastropods or bivalves

**Mollusks** include species of snails, slugs, squids, octopi, and clams. These species have true coelom that is covered by mesoderm tissue. They have various organ systems that also function together to maintain homeostasis.

Mollusks play an important role in the environment, in fact, one emerging use of mollusks is to check the quality of water. Since filter-feeder bivalves can concentrate pollutants and microorganisms in their muscle tissues, careful investigations of bivalves can give warning to biologists and public health officials about the dangers or pollution present in the water environment.

**Planarian** species locomotion is in a straightforward gliding movement via their cilia present in the ventral region of their body. While **earthworm** species move through the contraction of two specialized muscles, and the use of setae, prevents them from slipping from each other. Lastly, **scallops** move by clapping their shells quickly through the use of their body muscle. This muscle force the water towards their shell hinge, thus creating forward propulsion.

The slime’s sticky texture allows the snails to cling to surfaces while their muscles are at rest. As they move along, the slime makes their movement faster. This also helps them stay moist and protects their skin against harmful factors in the environment.

If a land snail would stop producing this mucus, climbing and clinging to surfaces will be very difficult for them. They would no longer have protection against environmental hazards that may harm them.

Both earthworms and clams are filter feeders. Earthworms use their pharynx to filter food particles and nutrients. The filtered particles go through the esophagus and into the digestive tract. On the other hand, clams feed by filtering the food using their gills. As the water enters through the siphon, it flows to the gills and exits again to the siphon. The plankton trapped by the gills moves into the mouth of the clam.

Earthworms dig holes into the soil, allowing the plants’ roots to have more breathing space and access to nutrients from the surface. Earthworms also break down compost and excrete valuable nutrients that plants use.

Bivalves that live in muds or sediments need siphons which can reach up to the surface of the sediment. This is because they need the water to respire, feed, excrete, and reproduce.

**Pearls** are produced by marine oysters as a natural defense mechanism against any irritant such as pathogens and sands that enter their shell or body. These irritants may cause damage to their fragile muscular body, hence the oyster secretes a protective layer of aragonite and conchiolin covering that envelopes the irritant. This is process is an advantage to the oyster’s life because it protects them against any danger.

If the optic glands in the octopus are removed, the life-span of the female octopus extends since its feeding will resume. This means that if the gland is there, the octopus will eventually die. However, if the gland is not there, it will live longer. This indicates that the hormones secreted by the glands affect the life-span and body functions of an octopus.

The nematode ***Caenorhabditis elegans*** is a free-living nematode used as a common model for genetic and cell studies. This organism is very suitable for scientific experiments, especially with those medically inclined research studies because most of the genes found in *C.elegans* genome have a similar functional counterpart to the human genome.

Also, the advantage of handling small size sample organisms would be an advantage for any study because the result can be generated in a small amount of time due to its lesser complexity.

The respiration of land-dwelling annelids depends on their moist skin. For example, earthworms exchange oxygen and carbon dioxide through their skin– their skin produces a thin protective mucus substance that keeps their outer covering from drying out. Therefore, during heavy rains, it is important for them to emerge from the ground because too much water in their habitat can disrupt their homeostasis and drown them.

The fluid that is secreted by the leeches contains an organic compound called hirudin. Hirudin prevents blood clotting by thinning out the blood or reducing its coagulation in the tissues. This allows the leeches to temporarily stop the clotting on the bitten area in order to get more blood.

To check if the pork is contaminated with Trichinella worms, look for any cysts in the intestinal walls or muscle tissues. These cysts are formed when the larvae burrowed themselves into the muscle tissues of the host.

The respiratory and circulatory system in mollusks depend on each other and work hand in hand in order to transport blood, oxygen, and carbon dioxide into all parts of their bodies. When mollusks undergo respiration, oxygen and carbon dioxide are carried into the bloodstream. However, mollusks may either have an open circulatory system that contains a heart that pumps blood and oxygen into an open cavity, or a closed circulatory system that uses arteries and veins to transport blood and oxygen.