Chordates are characterized by organisms that have at least a dorsal nerve cord, a notochord, pharyngeal pouches, and a tail.

D. a notochord

Similar to reptiles, birds lay amniotic eggs that are enclosed in a hard outer shell and membranes.

A. birds

Sharks, rays, and eels are classified as fishes. Turtles are members of the class Reptilia.

B. turtle

As ectotherms, they are able to regulate their body temperature through their interaction with the environment.

B. the environment

As endotherms, birds and mammals have a high metabolism rate which produces body heat. This heat keeps their bodies warm. In addition, the bird’s feathers and the mammal’s hair help them in insulating the body heat.

B. control body temperature from within

Aquatic chordates have acquired gills as an adaptation for living in an aquatic habitat. The gills facilitated in the gas exchange and allowed them to breathe underwater.

C. gills

Aquatic chordates have a single circuit circulatory system. It only has one atrium and one ventricle.

C. single-loop circulatory system

Another way for some animals to maintain homeostasis is to filter urea from blood, to excrete excess water, or to retain water through the use of the kidneys.

A. kidney

Animals that have a well-developed cerebrum is able to receive and interpret sensory information successfully. The cerebrum is responsible for the higher thinking functions of the brain.

B. cerebrum

Vertebrates are more mobile than other chordates due to their flexible backbone.

B. a flexible backbone

All nonvertebrate and vertebrate chordates share a common invertebrate ancestor. Chordates are characterized by organisms that have at least a dorsal nerve cord, a notochord, pharyngeal pouches, and a tail.

In tunicates, the notochord is lost when the embryo starts to mature. In vertebrates, the notochord is replaced by vertebrae once the embryo matures. In addition, young tunicates resemble a tadpole due to its tail and pharyngeal pouches (gills).

**Convergent evolution** is a type of evolution in which distantly related organisms (non-monophyletic) have evolved and adapted a similar trait due to their need. These specific adaptations help the species population to survive in their environment/habitat.

**Endotherms** are able to move around easily in the cold, however, they require more food to generate heat in their bodies **Ectothermic** are not able to move around in very cold environments, however, they require less food

The beginning of endothermy is still unknown. However, scientists hypothesized that the way animals perform temperature control or endothermy has evolved more than once according to the evolutionary history. There are pieces of evidence which suggested that the evolutionary lines of birds and mammals (endotherms) descended from reptiles (ectotherms).

The digestive system of vertebrate species has different specialized organs and mechanisms that enable them to adapt to their feeding habits. Vertebrate filter feeders have sieve-like mouth structures that enable them to strain microorganisms out of the water for food

Aquatic chordates have acquired gills as an adaptation for living in an aquatic habitat. The gills facilitated in the gas exchange and allowed them to breathe underwater. First, the gills take in the water and filter the oxygen. Then, the oxygen flows into the tiny blood vessels and into the cells. In turn, the gills forces out carbon dioxide using the water.

The **respiratory system** is composed of various organs that function in the exchange of oxygen and carbon dioxide in the body. The small balloon-like shaped structure present in the lungs called the **alveoli** is responsible for the movement of oxygen into the body, and the release of carbon dioxide out of the body. Each person has hundreds of thousands of alveoli located in their lungs. The alveoli increases the surface area of the lungs for the efficient gas exchange.

In a single circuit or single-loop circulatory system, it only has one atrium and one ventricle. This type of circulation allows blood to pass through the heart at once.

On the other hand, in a double-loop circulation, the blood goes into two loops or circuits (left atrium and right atrium). The right atrium receives deoxygenated blood from the veins that drain the organs of the body, while the left atrium receives the oxygenated blood coming from the lungs.

The **circulatory system** is an organ system that functions in the transport of nutrients including oxygen in the body as well as the pick-up of waste materials for excretion The main organ present in the circulatory system is the heart which functions as the *pumping* organ.

The number of heart chambers in a frog is three, two atrium and one ventricle. Fishes have two, one ventricle and an atrium. Reptiles have three chambers, two atria, and a partially separated ventricle.

Tunicates expel nitrogenous wastes in the form of ammonia. Since they are underwater, the fluid diffuses out of the siphons and goes into the surrounding water. On the other hand, other waste products such as uric acid are only expelled once a tunicate dies.

The main function of the **excretory system** is to eliminate the waste materials in the body. In vertebrate species, the main organ in this system is the paired kidneys. These kidneys are composed of many filtering cells that act to sieve those waste materials and return those useful materials to the body.

**Vertebrate** species have a high degree of cephalization in the body as the head is composed of a well-defined brain and spinal cord. The brain is divided into various regions and parts that perform specific functions for the maintenance of homeostasis.

Parts of the vertebrate brain:

1. Cerebrum – responsible for the thoughts, actions and higher thinking functions of the brain

2. Cerebellum – responsible for body movements, balance, and coordination

3. Medulla oblongata – responsible for the control and function of internal organs

4. Optic lobes – responsible for the function of the sense of vision

5. Olfactory lobes – responsible for the function of the sense of smell

Vertebrates are more mobile than other chordates due to their flexible backbone. Their vertebrae are connected by tough yet flexible ligaments. Because of this, their backbones are able to bend sideways, backward, and forward without breaking apart. This allows fishes and snakes to move like waves; amphibians and reptiles to create a stance like making push-ups and; mammals to stand and walk upright.

Both birds and mammals reproduce internally. The eggs are fertilized inside the female’s body. After fertilization, birds lay amniotic eggs, wherein the embryo is enclosed in a hard outer shell and membranes. On the contrary, mammals either lay eggs (monotremes) or give birth to a live young (placental mammals and marsupials).

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.

A. Diagram B illustrates a heart that has more carbon dioxide (blue) than oxygen (red) in the blood. It shows a single-looped circulatory system wherein the deoxygenated blood from the heart goes to the gills then to the rest of the body in a single flow.

B. Diagram A shows a heart with four chambers. It has two atria (left and right) and two ventricles that are completely divided.

C. Diagram C shows a three-chambered heart where oxygenated and deoxygenated blood mixes at the ventricle.

Whales are aquatic mammals that belong to order cetaceans of placental mammals. Moreover, fishes are chordates of primitive order. Both whales and fishes share the common habitat.

They both are not closely related, but share common features in relation to their body shape and absence of legs.

The similarities can be explained by convergent evolution. In such evolution, two different species are required to live in same ecological niche. They develop similar adaptive features and behavior, for their survival. Such adaptation will produce similar features in appearance and behavior.

Thus, fishes and whales share some common features.

Legs are used for locomotion or movement in animals. Amphibians and reptiles have legs that are attached to the backbone through muscles and ligaments. In this way, their appendages are jutted sideways and their movement is controlled.

On the other hand, mammals have legs that are situated under the backbone. It allows them to stand and walk upright using two legs. Because of this, mammals are able to support their body weight in a more efficient way compared to reptiles.

The **excretory system** is the main system that plays an important role in the elimination of waste, especially nitrogenous wastes in the body. The primary process in which the body removes these nitrogenous wastes is through dexifying them into a lesser toxic form (urea or uric acid).

**Ducks** are endotherms, which means they can generate and retain heat inside their bodies. On cold days duck sits on a sunny lawn with its wings outspread because the duck exposes its body to the sunlight. So that some heat may be generated from its body, this might help to maintain its body temperature.

But in hot summer, the duck sits in the shade of a tree with its bill open because in summer the external climate makes the duck lose the water present in the body. Hence it sits under the shade of a tree with its bill open so that it can take more gases into its body which might help the bird to maintain its body temperature. Both activities help the duck maintain homeostasis.

Vertebrate species show a high degree of cephalization where the brain is located in the head region. The brain controls various body processes including response to stimuli, motor voluntary movements, involuntary movements, and memory/learning. The doctor would suggest an X-ray of the dog’s brain to check if the brain has something to do with the problem in movement coordination.

The hair, fur, or feathers of endotherms help them maintain a stable internal temperature. It provides them insulation to enable them to thrive in a variety of habitats. To determine how do these adaptations keep an animal warm, you may conduct this experiment:

Materials:
a. wool yarn
b. feathers
c. jars with lid
d. hot water
e. cotton balls
f. freezer

Steps:
1. Prepare three jars and label it with A, B, and C.
2. Fill the jars with cotton balls.
3. Fill each jar with 3 cups of hot water.
4. Record the temperature in each jar.
5. Seal the jars. Set aside jar A.
6. Cover jar B with wool yarn.
7. Cover jar C with feathers.
8. Wait for 10 minutes then uncover all the jars. Record the temperature.
9. Repeat every 10 minutes until 30 minutes has passed.
10. Place all the jars inside the freezer.
11. Wait for 10 minutes and uncover the jars. Record the temperature.
12. Repeat every 10 minutes until 30 minutes has passed.
13. Tabulate all the temperature recordings for jars A, B, and C.
14. Compare the results.

A. Bony fishes and cartilaginous fishes
B. Reptiles and amphibians
C. Nonvertebrate chordates

The respiratory system and the circulatory system works hand in hand in transporting blood and oxygen into the body of a fish. First, the gills take in the water and filter the oxygen. Then, the oxygen flows into the tiny blood vessels. In these tiny blood vessels, the blood becomes oxygenated and it goes into the rest of the body. Once the oxygen is absorbed by the cells, the deoxygenated blood (with carbon dioxide) goes back into the heart then to the blood vessels in the gills. In turn, the carbon dioxide is diffused through the gills.