# Classification Based on Body Cavity (Coelom) The classification of animals is based on the presence and type of a **coelom**, which is a fluid-filled space between the outer body wall (derived from ectoderm) and the digestive tube (derived from endoderm). ## Types of Body Cavities Animals are categorized into three main groups based on the structure of their body cavity. ### 1. Acoelomate - **Definition:** Animals that have no body cavity. - **Structure:** The space between the ectoderm (outer wall) and endoderm (digestive tract) is completely filled with a solid packing of mesoderm tissue. - **Example Phylum:** Platyhelminthes (flatworms). ### 2. Pseudocoelomate - **Definition:** Animals with a "false" body cavity called a pseudocoelom. - **Structure:** The body cavity develops between the mesoderm and the endoderm. It is only partially lined by mesodermal tissue (it lines the ectoderm side, but not the endoderm side). - **Example Phylum:** Aschelminthes (nematodes/roundworms). ### 3. Coelomate - **Definition:** Animals with a true coelom. - **Structure:** The body cavity forms within the mesoderm and is completely lined by mesodermal tissue. This fluid-filled space is called the coelomic cavity. - **Example Phyla:** Annelida through Chordata. #### Comparison of Body Cavity Types | Feature | Acoelomate | Pseudocoelomate | Coelomate | | :--- | :--- | :--- | :--- | | **Body Cavity** | Absent | Present (Pseudocoelom) | Present (True Coelom) | | **Lining** | N/A (space is solid) | Partially lined by mesoderm | Completely lined by mesoderm | | **Example** | Flatworms | Roundworms | Annelids, Chordates | ## Divisions of Coelomates Coelomates are further divided into two major evolutionary lines based on their embryonic development, specifically the fate of the **blastopore** (the first opening formed in the embryo). ### 1. Protostomes - **Etymology:** From Greek, meaning "first, the mouth". - **Development:** The blastopore develops into the mouth of the animal. - **Examples:** Most Molluscs, Annelids, and Arthropods. ### 2. Deuterostomes - **Etymology:** From Greek, meaning "second, the mouth". - **Development:** The blastopore develops into the anus. The mouth forms later at a different location. - **Examples:** Echinoderms (sea stars, sea urchins) and Chordates (including vertebrates).

10.2 Classification Based Upon Type Of Body Cavity | Animalia | Biology 11