As a group, acoelomates are characterised by :

Options

A. a body cavity called a haemocoel
B. deuterostome development
C. a coelom that is not completely lined with mesoderm
D. a solid body without a cavity surrounding internal organs

Explanation

To solve the question regarding the characteristics of acoelomates, we can follow these steps:

### Step 1: Understand the term "acoelomate"
- Acoelomates are organisms that lack a coelom, which is a fluid-filled body cavity located between the digestive tract and the body wall.

### Step 2: Analyze the options provided
1. **A body cavity called a haemocyl**: This is incorrect because haemocyl is not a true coelom and acoelomates do not possess a body cavity.

2. **Deuterostome development**: This refers to a developmental process where the anus forms before the mouth. Acoelomates do not specifically exhibit this type of development.

3. **A coelom that is not completely lined with mesoderm**: This option describes a pseudocoelomate, not an acoelomate. Acoelomates lack any coelom.

4. **A solid body without a cavity surrounding internal organs**: This accurately describes acoelomates, as they have a solid body structure without a body cavity.

### Step 3: Identify the correct answer
- Based on the analysis, the correct answer is **"a solid body without a cavity surrounding internal organs."**

### Final Answer:
Acoelomates are characterized by a solid body without a cavity surrounding internal organs.

---