Direct Ancestors of Grade Radiata:

The direct ancestors of Grade Radiata are Protozoa, Parazoa, and Porifera. Let's explore each of these groups in detail:

1) Protozoa:
Protozoa are unicellular eukaryotic organisms that belong to the kingdom Protista. They are considered the most primitive group of eukaryotes and are believed to be the direct ancestors of all other multicellular organisms. Protozoa exhibit a wide range of structural and functional diversity.

Key Points:
- Protozoa are unicellular organisms.
- They are considered the most primitive group of eukaryotes.
- Protozoa exhibit a wide range of structural and functional diversity.

2) Parazoa:
Parazoa is a subphylum of animals that includes the phylum Porifera, commonly known as sponges. Parazoa organisms are considered to be the simplest multicellular animals. They lack true tissues and organs, and their cellular organization is relatively simple compared to other animals.

Key Points:
- Parazoa is a subphylum of animals.
- It includes the phylum Porifera (sponges).
- Parazoa organisms lack true tissues and organs.

3) Porifera:
Porifera, commonly known as sponges, is a phylum of aquatic animals. They are multicellular organisms that have specialized cells but lack true tissues and organs. Sponges are filter feeders, obtaining food particles from the water by pumping it through their bodies.

Key Points:
- Porifera is a phylum of aquatic animals.
- Sponges are multicellular organisms.
- They lack true tissues and organs.
- Sponges are filter feeders.

Summary:
The direct ancestors of Grade Radiata include Protozoa, Parazoa, and Porifera. Protozoa are the most primitive group of eukaryotic organisms and are considered the ancestors of all other multicellular organisms. Parazoa, which includes the phylum Porifera (sponges), represents the simplest form of multicellular animals. Sponges lack true tissues and organs but have specialized cells. This progression from Protozoa to Parazoa to Porifera provides insights into the evolutionary history of Grade Radiata.