An animal is a multi-cellular organism with cells that are not enclosed in a rigid cell wall. Animals differ from other organisms in two major aspects: 1) through the structure of their cell wall and (2) through their feeding habits. For example, unlike other organisms, such as plants, animals do not have a rigid cell wall. Plants on the other hand have a rigid cell wall.
In terms of nutrition, animals are heterotrophs because of the way they synthesize organic compounds. Plants, on the other hand, are autotrophs. Plants are classified as autotrophs because they synthesize organic molecules directly using energy from the sun to produce complex organic molecules. This process differs from the one in animals where they synthesize their organic molecules indirectly. As a result, animals obtain organic carbon by feeding on other autotrophs or heterotrophs.
Another way through which animals differ from other life forms is through specialization of cells. The specialized cells and tissues found in animals facilitate movement and nervous coordination. Lastly, animals differ from other life forms through sexual reproduction. Animals have specialized reproductive cells which undergo division (through meiosis) to produce smaller haploid gametes. The smaller haploid gametes then fuse to form a diploid zygote, which later develops into a new individual. Animals also go through several developmental stages before developing into a fully grown adult.
As indicated earlier, animals are multi-cellular. This characteristic is shared by many animals, and it is not found in non-animals. For example, Protozoa, Porifera and Cnidaria have inside and outside cellular layers. However, animals also some distinguishing features which can be found in one animal but lack in other animals. For example, sponges have a spongocoel which Cnidaria have a gastro-vascular cavity. In sponges, water enters through the spongocoel through the Ostia and exits the system through a large opening known as osculum. Cnidaria on the other hand have a gastro-vascular cavity which works as both the mouth and the anus. Substances that come in through the mouth exit through the same opening. Another difference also comes in the arrangement and function of cellular layers. For example, sponges have two layers: ectoderm and endoderm. The ectoderm acts as the protective outer layer, with channels leading to the collar cell. Some types of cnidarians, such as the sea anemones also have a specific numeric pattern: they have an octoradially symmetrical shape.
Moon Jelly (Aurelia aurita) is the most common type of jellyfish. They are easily recognizable from their pink crescent shaped gonads found on their underside. Mon Jelly has two main stages in its lifecycle: the polyp stage and the medusa stage. Mature polyps produce asexually though budding to form a colony of polyps. The polyps then reproduce through budding to form ephyra (small medusa). The medusae then swim off and mature. They then reproduce sexually. After the fusion of eggs and sperms from two medusae, a zygote is formed. The zygote then develops into planula (larva), which leaves the adult medusae to attach itself to a shaded surface. The planula then develops into a polyp, and the life cycle starts again.
Fasciola hepatica (sheep liver fluke) is a parasitic flatworm that infects livers of various animals, including humans. Fasciola hepatica is an important parasite of cattle and sheep. It causes fascioliasis, a disease found in areas with large herds of cattle and sheep. In order to complete its life cycle, F. Hepatica requires an intermediate host in which the parasite reproduces asexually. The reproduction cycle begins when adult flukes lay eggs in the bile ducts of the infected cattle or sheep. The eggs then enter the intestinal tract and are eventually expelled together with feces. When the feces drop into wet pasture, the eggs then hatch to release free-living miracidiae. The miracidiae then swim around to find an intermediate host snail. After finding a host, the miraciudm then multiplies asexually to produce cercariae. After two months, the cercariae then leave the host snail and swims around, finally attaching themselves onto grass. The cercariae then form a protective membrane, which enables to survive adverse climatic conditions (for as many as five months). As cattle and sheep graze, they then ingest metacercariae, which then hatches and develops into immature flukes. The immature flukes then move from the gastrointestinal tract towards the liver through the body cavity. Once they penetrate the liver, the immature flukes then migrate through the liver causing fascioliasis. After eight weeks, the immature flukes then enter the bile ducts where they develop into adults, feed on blood, lay eggs and cause acute fascioliasis.
Diphyllobothrium latum (fish tape worm) is the largest type of human tapeworm. It is transmitted to humans after the consumption of infected fish. Their reproduction cycle begins when immature eggs of D. latum are passed in feces from the host. When the conditions are right, the eggs mature into oncospheres. This takes place in 18-20 days. The oncospheres later develop into coracidia, which has the capacity to swim thus attracting potential intermediate host. After ingestion by an intermediate host, the coracidia then develop into precercoid larvae. After ingestion by a second intermediate host, the precercoid larvae are then released from the crustacean and migrate into the fish flesh where they develop into plerocercoid larvae. When humans consume infected fish, plerocercoids then develop into immature adults, and later into mature adult tapeworms. The tapeworms reside in the small intestine of the host. The adult tapeworms attach to the intestinal mucosa through two bilateral grooves.
One characteristic shared by Aurelia aurita, Fasciola hepatica, Diphyllobothrium latum is the ability to undergo both sexual and asexual reproduction. Asexual reproduction offers several advantages to these organisms in a number of ways. For example, the organisms can reproduce quickly using less energy. Again, the offspring look exactly the same. However, asexual reproduction has its own disadvantages as well. For example, the organism lacks genetic variation, which means that they can become extinct due to dramatic changes in the environment. Sexual reproduction on the other hand enables organism to differ genetically and slow down the population growth. However, it is also disadvantageous in that organisms use a lot of energy and is a slow process.
In the animal kingdom, some organisms are more advanced than others. For example, organisms belonging to phylum annelida are more advanced than organisms belonging to phylum platyhelminthes. Tiger Planaria (Dugesia tigrina) is a good example of an organism belonging to phylum platyhelminthes (flatworm) while Iridescent Phyllodoce (Phyllodoce multipapillata) is a good example of an organism belonging to phylum annelida.
Annelids are more advanced than flatworms in a number of ways. For example, annelids have a segmented body while flatworms are not segmented. Annelids have a distinct head, a body segment and a tail. Because of their segmentation, annelids (Phyllodoce multipapillata in this case) are able to develop different body regions for different purposes.
Therefore, segmentation allows for specialization of body regions for various purposes in Phyllodoce multipapillata. On the other hand, Dugesia tigrina does not have specific body parts for specific purposes because they are not segmented. Again, annelids spend less energy to move because they can contract a few segments for them to move.
Another advancement annelids have over flatworms is that annelids have two body openings while flatworms have one body opening. For example, Phyllodoce multipapillata have a mouth and anus. Dugesia tigrina on the other hand have one body opening only which acts as the mouth and the anus. The presence of two body openings ensures that Phyllodoce multipapillata can feed and excrete at the same time. For Dugesia tigrina, they can only feed or excrete at any particular point because they have one opening.
Annelids also have the capacity to transport oxygen to the various body parts because they posses red blood cells. The presence of red blood cells also facilitates gaseous exchange. Flatworms, on the other hand lack red blood. This means that gaseous exchange takes place over the skin surface. As a result, flatworms cannot transport oxygen to their body parts.
It is true that insects are the most successful group of animals on earth in terms of diversity, biomass and geographic distribution. Although there is no single ecological or physiological characteristic which can explain this success, insects have a combination of characteristics which give them distinct advantages for survival. Some of these attributes include small body sizes, compound eye structure, the presence of an exoskeleton, the capacity to fly, the ability to undergo complete metamorphosis and the capability to adapt to the changing environment.
Because of their small body sizes, insects require minimum resources in order to survive and reproduce. In some cases, the presence of a single plant or animal is enough to cover the food requirements of an insect throughout its entire life. Another attribute that gives insects a distinct advantage over other animals is their compound eye structure. The reason why it is called a compound eye structure is because it is made up of repeating units, each of which functions as a separate visual receptor. The compound eye structure enables insects to see in every direction, and this makes it possible to identify sources of food, potential enemies, and the best habitat places. The compound eyes consist of small units called ommatidia, with each pointing in a different direction. This makes it possible for insects to work out the direction of objects and their distance. Insects can also have dual- vision, and this means that they can see dark, light and color. For example, bees are able to differentiate between a mature flower and a dying one.
Another unique attribute found in insects is the ability to undergo metamorphosis. Insects go through various developmental stages before attaining maturity. In some primitive insects, the developmental changes occur gradually, while other advanced insects undergo complete metamorphosis. In gradual metamorphosis, the insects develop organs of flight and reproduction incrementally, with the being fully useful during the adult stage. In complete metamorphosis, insects undergo dramatic changes in form and function. For example, the larva stage is used for feeding and growth, and the energy is stored in reserves for use during the later stages in life. As the insect transforms from the larva to the pupa stage, massive reorganization takes place. The same thing also applies when the insects transforms from pupa into adult. The capacity to undergo complete metamorphosis ensures tha each stage is adapted to its ecological function, thus the insects are able to exploit different environmental resources, and colonize different habitats.
The presence of an exoskeleton in insects is another unique attribute which gives them a distinct advantage. The exoskeleton gives insects shape, and supports their soft body tissues. The exoskeleton also protects the insect from attack and injury. In some cases, the exoskeleton reduces the loss of water. This is because the exoskeleton is made up of an impervious layer of wax. The membranes of the exoskeleton also aid in supporting movement. Apart from the exoskeleton, insects also have the capacity to fly. The presence of wings makes flight possible. The wings of insects are very powerful because they can deliver twice more power per unit of muscle mass compared to birds. This ensures that insects use their energy efficiently. The ability to fly enables insects to escape from predators, and move to new habitats.
Organisms found in phylum mollusca are the second most diverse group of animals after those found in phylum anthropoda. The reasons for this success can be explained by a number of factors. For example, the presence of abalone shells, found in unique type of snails. The abalone shells have a wide opening for the body, and holes along the left side of the shells. The holes on the side of the shell continue to form even into adulthood. The holes perform important functions in that they facilitate respiration, reproduction and sanitation of abalones. The formation of the shell begins at the larva stage and the abalone adds onto the existing shell after this stage. When the abalone is removed from the shell, it can continue to exist but cannot make a new shell. One of the advantages of the shell is protection from potential predators.
Another unique feature of the organisms found in phylum mollusca is their diverse feeding mechanisms. For example, gastropods use their radula to feed; they drill holes into the shells of other organisms and use the opening to feed. Digestion then takes place in the posterior part, and waste materials leave through the anterior part. In bivalve molluscs, they have sharp teeth to enable them scrape algae from rocks, and have the capacity to filter fine particles from water. Their whelks also have a radula, which facilitates boring shells of other animals and to suck out flesh from their victims. In other molluscs, such as the cephalopods, they have suckers for grabbing and holding objects. Their mouths are also armed with a very sharp beak, made of two pointed plates. Their mouths also produce poison to paralyze their prey.
Molluscs also have the capacity to control buoyancy. For example, nautilus has small tubes near the its tentacles for expelling water when under pressure. This enables nautilus to move at high speeds in the opposite direction. Other mollusks achieve buoyancy by allowing pure water into their kidneys, and this forces salty water out of their body cavities.
Octopuses achieve buoyancy by having a gelatinous body, which allows light chlorine ions to replace sulfate in their bodies. This makes them slightly denser than the sea water and facilitates movement.
Another feature that facilitates survival of molluscs is the capacity to change color. For example, squids have the capacity to camouflage, and this confuses their undersea activities. This is facilitated through a special pigment in their skin, which can vary their color and patterns. By changing their color patterns to match the sea floor, squids are able to escape from their predators.
Bailey, Jill. Animal life. Wellington : Oxford University Press , 1995. Print.
Barker, G. M. The Biology of Terrestrial Molluscs. Wallingford : CABI, 2001. Print.
Richardson, Adele. Insects. Mankato, MN : Capstone , 2004. Print.
Taggart, Ralph, Christine Evers and Lisa Starr. Biology: Diversity of life, 13 ed. . Stamford,
CT : Cengage Learning, 2011. Print.
Russell, J. Peter, Hertz, Paul and McMillan, Beverley. Biology: The Dynamic Science.
Stamford, CT: Cengage Learning, 2010. Print.