All of the following statement are correct except.

Options

A. In Balanoglossus,excretion takes place through proboscis gland
B. All vertebrates are chordates but all chordates are not vertebrates
C. Placoid scales are found in Osteichthyes
D. Mammary glands are defining character of all mammals

Answer

To solve the question, we need to analyze each statement provided and identify which one is incorrect.

### Step-by-Step Solution:

1. **Statement 1: In Balanoglossus, excretions take place through the proboscis gland.**
- **Analysis:** This statement is correct. The proboscis gland in Balanoglossus is indeed responsible for excretion. It is located in the frontal portion of the organism and excretes waste products in the form of granules.

2. **Statement 2: All vertebrates are caudates, but all caudates are not vertebrates.**
- **Analysis:** This statement is also correct. All vertebrates belong to the phylum Chordata (which includes caudates), but not all caudates are vertebrates. For example, Urochordata and Cephalochordata are part of Chordata but are not classified as vertebrates.

3. **Statement 3: Placoid scales are found in osteocytes.**
- **Analysis:** This statement is incorrect. Placoid scales are found in cartilaginous fish like sharks and rays, not in osteocytes (bony fish). Osteocytes are associated with bony structures and do not have placoid scales.

4. **Statement 4: Mammary glands are a defining character of all mammals.**
- **Analysis:** This statement is correct. Mammary glands are indeed a defining characteristic of mammals, as they produce milk to nourish their young.

### Conclusion:
The incorrect statement is **Statement 3: Placoid scales are found in osteocytes.**

### Final Answer:
The statement that is incorrect is: **"Placoid scales are found in osteocytes."**

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