Choices A, C, and D are incorrect. The term stimulus is defined as any detectable thing or event from the environment that triggers a response from an organism.

B. stimulus

Choices A, B, and D are incorrect. The simplest nervous system is a nerve net. Nerve nets consist of neurons that show a net-like arrangement. They are spread throughout the entire body of the organism.

C. nerve nets

Choices A, B, and C are incorrect. The region of the brain that is responsible for movement and balance is the cerebellum.

D. cerebellum

Choices B, C, and D are incorrect. The arrows in the diagram are pointed to the ganglia. The term ganglia are used for describing the group of interneurons that are connected or linked with each other.

A. ganglia

The two trends in the evolution of the nervous system of invertebrates are cephalization and specialization. Cephalization is the concentration of sensory organs and nerve cells in the head, which is the anterior part of the body. On the other hand, specialization refers to the way neurons are organized into groups to perform complex functions.

From the fishes to mammals, the brain has evolved in terms of its size and complexity. When compared to other animals, mammals the largest brain with a well-developed cerebrum, which is the thinking region. In addition, the cerebellum of mammals is more developed than the ones found in fishes, amphibians, and reptiles. Having a well-developed brain has caused mammals to perform a complex behavior and learning.

The environmental stimuli that only the animal’s sensory system are able to detect or sense include the weak type of electric currents, heat vision, pheromones, and the Earth’s magnetic field.

Choices A, B, and C are incorrect. A hydrostatic skeleton is made up of fluids contained in a body cavity and it allows the organism to change its body shape by contracting its body walls. This type of skeleton is seen in annelids and cnidarians.

D. annelid

Choices A, B, and D are incorrect. The endoskeleton of vertebrates are made up of two tissues–bones and cartilage that give structural support to the body.

C. cartilage and/or bone

Choices A, C, and D are incorrect. When the muscles are stimulated by the nervous system, they start to contract or shorten. The contraction of the muscles produces physical force.

B. only when they shorten

Fishes move their body in a wave-like motion in order to swim through the water. They contract and expand their bodies to move forward. First, they expand the muscles on one side of the body, whereas the muscles on the other side are relaxed.

Joints are the link between bones and enable body movements. Without the joints, vertebrates would not be able to move and perform tasks, such as jumping and running.

Moving and bending the knees occur when the muscles pull the bones at a joint. However, they do not have the ability to push the bone back to its original position. In this case, muscles work in pairs–flexors and extensors. Flexors are the muscles that contract to bend a knee. On the other hand, the extensors are the ones that stretch or straighten the same knee.

Choices A, B, and D are incorrect. Internal fertilization, asexual reproduction, and amniotic eggs are the reproductive features which are observed in vertebrates. Budding is a type of asexual reproduction that is commonly used by invertebrates, such as bacteria and corals.

C. budding

Choices B, C, and D are incorrect. Oviparous species have embryos that develop outside of the parent’s body.

A. oviparous

Choices A, B, and D are incorrect. Female mammals have mammary glands in their breasts. These glands secrete milk to feed the young offspring.

C. mammary gland

Cnidarians have two body forms in its life cycle–medusa and polyp stages. First, the polyp reproduces asexually through budding in order to produce young medusas. Then, the male and female medusas would swim freely in the water and release haploid gametes (sperm and eggs). Next, the gametes fertilize in the open water to form a diploid zygote. Eventually, the zygote grows into a larva, which attaches into the surface. and develops into a polyp. Later on, the life cycle starts again.

The animal’s eggs can be fertilized in two ways: internal and external fertilization. These two types of fertilization occur when the sperm fuses with the egg cell to produce an offspring. However, these methods differ in some ways. Internal fertilization takes place inside the body of the female or the egg-producing individual. On the other hand, external fertilization takes place outside the body of the female or the egg-producing individual.

Placental mammals have an embryo that develops inside their body with the help of the placenta for nourishment. In terms of survival advantages, the placenta is quite helpful in ensuring that the embryo would fully develop before they are born. Since the offspring inside the mother’s womb stays there for a long time, they are born at an advanced stage of development.

Choices B, C, and D are incorrect. The term homeostasis refers to the organism’s state of stability when it comes to its internal conditions, which are often maintained by physiological processes. This allows all living things to adjust to the conditions that are favorable for their survival.

A. homeostasis

Choices A, C, and D are incorrect. Ectotherms maintain their body temperature with the help of the factors found in the external environment. Since they do not have body insulation features, they regulate their temperature by absorbing heat from the sun when the climate is cold and by using underground burrows to release heat when the climate is hot.

B. the environment

Choices A, C, and D are incorrect. Endotherms have the ability to regulate their body temperature internally. They have high metabolic rates and body insulation features, such as feathers, body fat, and hair, that help in conserving body heat when the climate is cold. When the temperature in the environment is hot, they either pant or sweat to release heat.

B. control body temperature from within

The endocrine glands are responsible for releasing specific hormones into the bloodstream that would help regulate the activity of cells and organs. In this way, homeostasis is maintained and metabolism is regulated.

The function of the endocrine system is to control the body functions by producing hormones that would drive the physiological processes and to maintain internal balance in our body. It interacts with the other body systems to ensure that the body is healthy, stable, and highly functional.

Here are the advantages and disadvantages of ectothermy and endothermy:

1. Ectothermy
Advantage: Since their cellular processes are slow, they conserve more energy; hence, they need less food than endotherms.
Disadvantage: They are vulnerable to changes in the environment’s temperature.

2. Endothermy
Advantage: Their reproductive and survival rates are high because they can adapt to the fluctuations in the environment’s temperature.
Disadvantage: Since they have high metabolic rates to keep their bodies warm, they use a lot of energy. Therefore, they require more food than ectotherms.

According to the scientists, the evolution of endothermy happened twice among vertebrates. The first evolution happened in ancient reptiles (ectotherms) that evolved into birds. The second one occurred when the ancient reptiles evolved into mammals.

Movements occur when the muscles pull the bones at a joint. However, they do not have the ability to push the bone back to its original position. In this case, muscles work in pairs–flexors and extensors. Flexors are the muscles that contract to bend a limb or other parts of the body at a joint. On the other hand, the extensors are the ones that stretch or straighten the same part.

The major types of neurons are the following:

a. Sensory neurons: They are the specialized cells that respond to the stimuli that are detected by the different sensory organs, such as eyes, nose, ears, tongue, and skin.

b. Motor neurons: They are assigned to carry instructions from the interneurons to the muscular system. They trigger the muscle tissues to generate a response, particularly a movement when the body detects a stimulus.

c. Interneurons: These are the neurons that process and transmit information to other neurons or cells.

The region of the brain that is responsible for movement and balance is the cerebellum. In this case, the veterinarian might suspect that is a problem with the animal’s cerebellum if the dog shows poor coordination.

Here are the advantages and disadvantages of exoskeleton and endoskeleton:

1. Exoskeleton
Advantages: It serves as a great defense against predators and other environmental conditions, such as rain and sunlight. It also gives protection to the animal’s soft organs and prevents injury.
Disadvantages: It is rigid and heavy; hence, it reduces the animal’s mobility and flexibility. In addition, the exoskeleton does not grow together with the organism. In this case, the organism undergoes molting and becomes defenseless for a certain period of time.

2. Endoskeleton
Advantages: The presence of exoskeleton gives structural support to the body. It can support weight without making it too heavy for the organism. Unlike exoskeleton, it grows together with the organism and does not require molting.
Disadvantages: It is prone to breakage and offers less protection to the organism. In addition, not all areas are rigid so it causes the muscles to have lesser leverage.

Placental mammals produce an offspring that develops inside the mother with the help of the placenta for nourishment. After birth, the mother cares for their young by nursing them.

On the other hand, marsupials lack a placenta. Instead, they have a marsupium, which is a protective pouch found in female marsupials, to nourish their newborn young. After birth, they are carried in this pouch in order to nourish and to continue its growth and development.

The invertebrate in the diagram shows a hydrostatic skeleton. It is made up of fluids contained in a body cavity. It allows the organism to change its body shape by contracting its longitudinal muscles along the body walls.

The parental behavior of mammals is like an investment, wherein the parents use energy in helping their offspring to develop in a competitive environment until they are ready to be on their own. Due to environmental stability, predation risks, and frequency of reproduction, this behavior has been naturally selected for the mammals. This kind of behavior is something that relates to kin selection since it increases the genetic fitness or survival rate of related species.

The two body systems that interact to deliver hormones in the body are the endocrine and circulatory systems. When the body detects a stimulus, the information is transmitted into the endocrine system. In turn, the body reacts to the stimulus by secreting hormones from the endocrine glands. These hormones, which trigger various cellular activities, are released through the circulatory system, particularly in the bloodstream. The blood vessels become the channel in which the hormones are brought to different organs and cells.

Bird and mammals are endotherms; hence, they are able to produce body heat internally by having a fast metabolism. In addition, the bird’s feathers and the mammal’s hair are used to conserve their body heat. Because of this, they are able to survive both warm and cold biomes.

On the other hand, reptiles and amphibians are ectotherms. They are not able to reproduce heat internally due to their slow metabolic rate. Instead, they are dependent on the environment as the source of their body heat; hence, they are more likely seen in warmer biomes than in cold ones.

The label A in the diagram represents the amnion. This is a sac that is filled with an amniotic fluid, which serves as a watery cushion that protects the embryo during its development.

The label B in the diagram represents the yolk sac. This membrane surrounds the yolk, which is the food and nutrient source of the embryo.

The exoskeleton of flies serves as a great defense against predators and other environmental conditions, such as rain and sunlight. It also gives protection to the animal’s soft organs and prevents injury.

However, the exoskeleton is rigid and heavy; hence, it reduces the insect’s mobility and flexibility when it grows bigger. In addition, it does not grow together with the organism. In this case, when the insect outgrows its exoskeleton, it undergoes molting and becomes defenseless for a certain period of time.

The nervous system is compared to a network of wired telephones. Similar to a telephone network, the nervous system receives and transmits information from the brain. The telephone company is like the brain, which is the command center where the role and tasks of all the parts of the body are given. On the other hand, each telephone in our houses and offices receive and send information just like the nerve cells. The wires of the telephone are like the interneurons that transmit the impulses between sensory and motors neurons.

One example of the cause-and-effect relationship that is involved in maintaining the homeostasis is controlling the glucose level in the body. Glucose is a sugar, which is an important source of energy, that is found in our blood. Our body maintains normal glucose levels to ensure that we are healthy. When glucose levels are too high, it signals the pancreas to release a hormone known as insulin. This hormone instructs the liver, muscles, and fat cells to use the glucose as energy. However, once the blood glucose levels begin to normalize, the pancreas would stop making insulin.

When the environmental temperature increased, the body temperature of a pigeon, which is an endotherm, has increased very little. Its body temperature shows a relatively narrow range, which indicates that their temperature control is better than alligators, snakes, and lizards.

On the other hand, the body temperature of the lizard, which is an ectotherm, increased along with the changes in environmental temperature. Their body temperature shows a relatively wide range; hence, their temperature control is not as good as endotherms.

The classification of animals in the graph are as follows:
a. Ectotherms: lizard, snake, alligator
These animals are classified under this group because their body temperatures show a relatively wide range. The increase/decrease in the body temperature was caused by the changes in the environmental temperature.

b. Endotherms: rabbit, cat, pigeon
These animals are classified under this group because their body temperatures show a relatively broad range. Even if there is an increase or decrease in the environmental temperature, their body temperature changes in a steadier rate than the ectotherms.

A sample graph below shows the temperature control of a lizard. A linear equation that can be used to relate the body temperature of lizard to its environmental temperature is y = 1x + 0, or simply y = x. The range at which the equation can model the situation would be all real numbers except for zero.

A sample graph below shows the temperature control of a lizard. A linear equation that can be used to relate the body temperature of lizard to its environmental temperature is y = 1x + 0, or simply y = x. The range at which the equation can model the situation would be all real numbers except for zero. (Click to see the graph)

Humans have a minimum frequency of 30 Hz and a maximum frequency of 23,000 Hz. This shows a mid-range hearing compared to other animals. They have a stronger hearing than the tree frog, canary, and elephant. However, they have a weaker hearing when compared to the dog, bat, and dolphin.

The wide hearing range of dolphins and bats enable them to conduct echolocation. This is a process wherein an animal emits a sound and listens to the echo that returns near them in order to navigate in various environments. Echolocating animals rely on their hearing, which is why they have a heightened hearing ability that can detect sound waves that are way beyond of what a human ear can normally hear.

The three main types of a skeleton are the hydrostatic skeleton, the exoskeleton, and the endoskeleton. In order to classify animals according to their skeleton, we can observe their characteristics and see if any of them falls under these categories:

a. Hydrostatic skeleton: It is made up of fluids contained in a body cavity. This type of skeleton allows the organism to change its body shape by contracting its body walls. It is usually seen in soft-bodied animals such as cnidarians, earthworms, sea anemones, and some invertebrates.

b. Exoskeleton: It is made up of chitin or calcium carbonate and serves as the organism’s outer body covering. Crustaceans and insects have this hard exoskeleton in their bodies.

c. Endoskeleton: It is made up of calcified plates, bone, and cartilage that give structural support to the body of organisms. This type of skeleton is found inside the body of the organism. Organisms that have this type of skeleton are sponges, echinoderms, fishes, reptiles, amphibians, birds, and mammals.

Sexual reproduction increases the genetic diversity in the organisms. This allows a population to have survival and reproduction advantages during natural selection. However, sexual reproduction has a fallback since the frequency of reproduction is low. It takes longer to produce an offspring in this type of method than asexual reproduction.

On the other hand, asexual reproduction involves the production of offspring that are genetically identical to the parent. This method allows a population to produce rapidly in large numbers. However, its fallback is the decrease in the genetic variability among species and in the overall genetic diversity in organisms.

The importance of maintaining homeostasis in an animal is to ensure its good health. Ensuring the stability of the body’s internal conditions prevents the organ systems to malfunction. More importantly, it raises the ecological fitness of an individual.

In the event that an organism is not able to maintain homeostasis, the functions or processes of the enzymes, cells, and organs would be disrupted. If this abnormal body condition is prolonged, the animal will become sick. In worst cases, it may even cause death.