The remnants of the Cambrian Explosion during the early Cambrian Period were discovered as the ancestors of the most modern animal phyla.

B. Cambrian Period

A diagram that is used to represent the relationships among organisms in terms of their evolution history is called a cladogram.

A. evolutionary relationships

Pseudocoelom is a body cavity that lies between the endoderm and mesoderm tissues of a roundworm. Therefore, roundworms are called pseudocoelomates.

D. pseudocoelomates

Sponges digest their food inside the cells using intracellular digestion.

A. sponge

The organ system showed in the diagram is an arthropod’s nervous system. It has a centralized brain in the head, which is connected to a network of ganglia and sensory organs.

D. nervous system

To facilitate the respiration of an animal, the respiratory organs must contain moist and large surface areas.

D. moist

A closed circulatory system uses blood vessels such as arteries and veins to transport blood and oxygen throughout the body.

B. remains within blood vessels

Some insects have a saclike organ called the Malpighian tubules. This organ converts ammonia into the form of uric acid.

C. uric acid

Cnidarians have the simplest nervous system which consists of nerve cells that are called nerve nets.

C. nerve nets

An animal that has both male and female sex organs is called a hermaphrodite. This condition allows the organism to undergo self-fertilization.

B. hermaphrodites

**Burgess shale fossils** show that these organisms generally had body symmetry segmentation, a skeleton, a back and front end, and appendages that were adapted for many functions. Burgess shale organisms are very similar to most invertebrates found today.

The specialized cells create a larger and more complex form of tissues, organs, and organ systems that functions to carry out complex and distinct functions. Specialized cells may have arisen from mutations and various adaptive mechanisms which changed the evolutionary landscape. Without the formation of specialized cells, multicellular would not function and live as it should.

Organisms in **phylum Porifera** do not have body symmetry like some other invertebrates. A common example of an organism in phylum Porifera are sponges, which have no left or right sides and no “back” end or “front” end.

In cephalization, an organism’s sensory organs and nervous system is concentrated at the anterior end of the body. This produces the head and the brain in an animal. Because of this, animals become smarter since they have a better way to respond to external stimuli.

Here are the three germ layers of an animal embryo:

1. Endoderm – inner layer; develops into the inner lining of the organs

2. Mesoderm – middle layer; develops into the organs

3. Ectoderm – outer layer; develops into the exoskeleton or skin

Here are the differences between acoelomates, coelomates, and pseudocoelomates:

Acoelomates do not have coelomate, which is a fluid-filled cavity that is lined by the body tissues.

Coelomates have a true coelom surrounded by body tissues which protect the internal organs.

Pseudocoelomates have mesoderm tissues that partially line the body cavity rather than a true coelom.

Intracellular digestion takes place within the cells. The food particles are broken down and digested inside the cell. On the other hand, extracellular digestion occurs outside of the cells. The digestive system is responsible for digesting the food mechanically and chemically. Later on, the digested substances are absorbed by the cell.

A one-way digestive system allows the evolved animal to digest food more efficiently. A one-way digestive has specialized sections for processing food along the digestive tract.

Two examples of terrestrial invertebrates respiration are the spiders book lungs, which are made of parallel sheet like layers of thin tissue that contains blood. Another is the land snails inner mantle cavity, which is a moist large surface area lined with blood vessels

Some mollusks, such as snails and clams, have an open circulatory system that contains a heart that pumps blood into blood vessels that lead to an open cavity. In this type of circulation, the blood surrounds the tissues and organs.

On the other hand, annelids such as earthworms have a closed circulatory system that uses arteries and veins to transport blood and oxygen in their body.

The three forms of nitrogenous wastes:

A. Ammonia: Aquatic invertebrates excrete wastes in the form of ammonia.

B. Urea: Terrestrial invertebrates excrete urea from the body in the form of urine.

C. Uric acid: Insects and arachnids have a saclike organ called Malpighian tubules that expels uric acid and other digestive wastes in the form of a thick paste.

The nervous system of modern invertebrates exhibit centralization, cephalization, and specialization.

Centralization means that the individual nerve cells form a network that is connected to the central nervous system.

Cephalization means the concentration of sensory organs and nerve cells are located in the anterior part of the body.

Specialization means that there is a variety of sensory organs and nerve cells assigned to perform specific tasks.

Water is passed through the large surface area of the gills that are rich in blood vessels. This enables the aquatic animal to exchange gases from the water to the blood vessels. This enables the aquatic animal to breathe efficiently

The **circulatory system** is the system that functions in the transport of nutrients including oxygen in the body as well as the pick-up of waste materials for excretion. One component of the circulatory system is the **heart**.

The **heart** is the main organ of the circulatory system. It is composed of muscle fibers and it is divided into four main chambers, powered and sustained by electrical impulses. It is the primary organ responsible for pumping and transport of blood and transportation of nutrients in the body.

**Internal fertilization** is a type of fertilization that happens inside the body of one parent. This is the typical fertilization in land animals including the human species. For this fertilization to occur, male sperms should be introduced inside the female reproductive tract.

**External fertilization** on the other hand, is the release of sperm and egg cells into an external environment usually in water or a moist environment, in which the sperm can fertilize the egg outside the female reproductive tract. One example organism doing external fertilization are **frogs**.

Multicellular animals should control the amount of water in their tissues and at the same time must release and excrete ammonia in their body system. The excretory mechanism of invertebrate species varies in many ways. For example in terrestrial invertebrates in order to preserve water and dispose ammonia, they are turning this first into urea which will then be eliminated along with urine.

For aquatic invertebrates, ammonia freely diffuses from their body tissues into the water environment. The water then immediately dilutes the ammonia. In some species like in Planarians, there is a presence of **flame cells** that eliminate the excess water. The water travels through this and leaves through tiny pores in the flatworm’s skin.

The slug slime allows them 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 and other hazards found in the environment.

**Bilateral symmetry is an important development in the evolution of animals.**

1. In invertebrates with bilateral symmetry, sense organs and nerves tend to be concentrated in front of their body. Such invertebrates can respond better to environmental stimuli.

2. Invertebrates having bilateral symmetry have segmented bodies. In the course of evolution, these segments evolved to become specialized cells and organs. Such specialized cells are able to perform specific functions.

3. Segmentation also helps an organism to increase its body size without the use of much genetic material. This happens as a result of the repetition of similar cells in the segments.

Aquatic invertebrates excrete wastes in the form of ammonia. Since they are underwater, the fluid diffuses out of the animal’s skin into the surrounding water. However, when invertebrates moved to terrestrial, they have acquired several adaptations in their excretory system to survive their new environment.
Terrestrial invertebrates excrete wastes from the body in the form of urine. Since terrestrial animals live on land, they need to conserve the water inside their body. The urine that they discharge contains a high concentration of urea and only a little amount of water.

Invertebrates make use of a variety of structures for respiration. All these respiratory surfaces have two common features. All these surfaces are moist to facilitate the easy diffusion of gases across the membrane. Also, they are thin and have a large surface area.

The larger the surface area, the better and the more the number of gases exchanged. Each type of vertebrate has different structure for respiration. Some invertebrates like worms respire through skins. Mollusks. annelids and some arthropods respire via gills. The gills are very different than skin. Terrestrial invertebrates like snails make use of mantle cavity to respire. Spiders have book lungs, whereas insects respire through spiracles.

Segmentation is an essential character in the development of animals because it allows animals to increase in body size with new genetic materials. Over the course of evolution different segments have become specialized for specific functions.

If this character called segmentation had not occurred in animals we might not find different organs in the body. The whole animal might have been a single tissue-like substance that may or may not perform all the functions required by an animal. This might have been a negative impact on the evolution of the animal kingdom.

The name for this type of skeleton is called hydrostatic skeleton. Some invertebrates, such as cnidarians, have this type of skeleton. In a hydrostatic skeleton, muscles surround a body cavity surrounding fluid supporting those muscles. When the muscles contract or expand, they push against the body cavity, which causes the body to change shape, because the fluid-filled cavity supports the muscles.

According to the evolutionary history of invertebrates, the arthropods share several features with annelids. Here are the similarities between arthropods and annelids:

1. Both arthropods and annelids have segmented bodies that exhibit bilateral symmetry.

2. Members of the annelid class Polychaeta have a pair of appendages on each segment, which resembles the appendages in some arthropods.

3. There is a similarity in the way the nervous system appears in arthropods and annelids. Both groups exhibit centralization and cephalization.

4. Both arthropods and annelids show a tubular, dorsal heart.

5. In terms of the presence of coelom, both are coelomates.

6. Both are protostomes, which means that they both have a one-way digestive tract.

Flame cells are responsible for getting rid of waster and excess water in a flatworm. If these type of cells are damaged, nitrogen-containing wastes will not be removed and ammonia will start to accumulate inside the body. In addition, the amount of water inside the cell would no longer be controlled. As a result, the flatworm may die since it can no longer survive the presence of high level of toxin and water in its body.

During evolution, the appearance of animals took place in the following stages. Initially it was multicellularitv which later on formed tissues. The next developmental stage was protostome formation, which helped in the formation of pseudocoelom in animals like round worms. In further development a true coelom formation took place which helped in the development of deuterostomes like echmoderms.

Both flatworms and insects can detect the intensity of light. Flatworms have two eye-spots that are able to detect light. Since they have this feature, they tend to move away from light. On the other hand, the insects have compound eyes. Their compound eyes enable them to see both light and dark in the form of a mosaic image. This allows insects to have an excellent advantage in detecting motion.