Here is a brief description of the evolution of the different body systems of invertebrates:

1. In feeding and digestion, from a simple intracellular digestion, more complex invertebrates are now using extracellular digestion.

2. In respiration, respiratory organs contain moist and large surface areas.

3. In circulation, blood is circulated throughout the body by an open or a closed circulatory system.

4. In excretion, controlling the amount of water and getting rid of wastes are performed by the excretory system.

5. In response, the nervous system processes the stimuli from the environment and instructs the body how to react. There is a presence of cephalization and specialization of cells and sensory organs.

6. Invertebrates have three kinds of skeletal system which include hydrostatic skeletons, exoskeletons, and endoskeletons.

7. In reproduction, both sexual and asexual reproduction are exhibited by invertebrates.

The advantage of asexual reproduction allows animals to take advantage of favorable conditions and the disadvantage is that there has no biodiversity, for parents and offspring have identical genes. The advantage of sexual reproduction is that offsprings have combinations of different genes that allow biodiversity. The disadvantage of sexual reproduction is that eggs may fail to get fertilized due to environmental conditions.

Arthropods 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 have a closed circulatory system that uses arteries and veins to transport blood and oxygen in their body.

Invertebrates have three kinds of skeletal system which include hydrostatic skeletons, exoskeletons, and endoskeletons.

Hydrostatic skeletons consist of a fluid-filled cavity that is surrounded by muscles.

Exoskeletons consist of a tough body covering made up of chitin and other proteins.

Endoskeletons are located inside the body. They are responsible for giving structural support and protection to an animal’s body.

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.

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.

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. In this way, water is not easily lost in the animal’s body.