The statement which does not stand true for arthropods is

Options

A. They have an open ciculatory system
B. Their body is divided into head, thorax and abdomen
C. They are segmented and coelomate animals
D. Fertilisation is usually external in them

Explanation

To determine which statement does not hold true for arthropods, we will analyze each statement one by one:

1. **Statement 1: They have an open circulatory system.**
- Arthropods possess an open circulatory system, where the blood is not always contained within blood vessels. Instead, it flows freely through cavities in the body. This statement is true.

2. **Statement 2: Their body is divided into head, thorax, and abdomen.**
- The body of arthropods is indeed divided into three main parts: the head, thorax, and abdomen. This segmentation is a characteristic feature of arthropods. This statement is also true.

3. **Statement 3: They are segmented and coelomate animals.**
- Arthropods have a segmented body structure, which is a key characteristic of this phylum. However, while they are segmented, they are not considered true coelomates as their body cavity is not lined with mesodermal tissue in the same way as in true coelomates. Instead, they have a hemocoel. Therefore, this statement is somewhat misleading but can still be considered true in the context of segmentation.

4. **Statement 4: Fertilization is usually external in them.**
- Most arthropods exhibit internal fertilization rather than external fertilization. This statement is incorrect as it misrepresents the typical reproductive strategy of arthropods.

Based on this analysis, the statement that does not stand true for arthropods is **Statement 4: Fertilization is usually external in them.**

### Summary of the Solution:
- **Statement 1**: True (open circulatory system)
- **Statement 2**: True (body divided into head, thorax, and abdomen)
- **Statement 3**: True (segmented, but not true coelomates)
- **Statement 4**: False (fertilization is usually internal)

**Final Answer**: Statement 4 is the statement that does not stand true for arthropods.