To determine which statement about Spongilla, leech, dolphin, and penguin is correct, we will analyze each statement provided in the question step by step.
### Step 1: Analyze the First Statement
The first statement claims that "penguin is homeothermic while the remaining three are poikilothermic."
- **Homeothermic** refers to animals that maintain a constant body temperature regardless of the environment (e.g., mammals and birds).
- **Poikilothermic** refers to animals whose body temperature varies with the environment (e.g., reptiles, amphibians, and most fish).
- **Analysis**: Penguins are homeothermic, but dolphins (which are also mammals) are homeothermic too. Therefore, this statement is incorrect.
### Step 2: Analyze the Second Statement
The second statement claims that "leech is a freshwater form while all others are marine."
- Leeches are indeed freshwater organisms.
- However, Spongilla (a type of sponge) is also a freshwater organism.
- **Analysis**: Since both leech and Spongilla are freshwater organisms, this statement is incorrect.
### Step 3: Analyze the Third Statement
The third statement claims that "Spongilla has special colloid cells called choanocytes not found in the remaining three."
- **Choanocytes** are specialized cells found in sponges (like Spongilla) that help in water filtration and feeding.
- The other three organisms (leech, dolphin, and penguin) do not have choanocytes.
- **Analysis**: This statement is correct as choanocytes are unique to sponges.
### Step 4: Analyze the Fourth Statement
The fourth statement claims that "all are bilaterally symmetrical."
- Bilateral symmetry means that the body can be divided into two identical halves.
- Sponges, including Spongilla, are asymmetrical, meaning they do not have a defined shape.
- **Analysis**: This statement is incorrect because Spongilla is asymmetrical.
### Conclusion
After analyzing all the statements, the only correct statement is the third one, which states that "Spongilla has special colloid cells called choanocytes not found in the remaining three."
### Final Answer
The correct statement is: **"Spongilla has special colloid cells called choanocytes not found in the remaining three."**
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