# 10.1 Criteria for Animal Classification The primary criteria for classifying animals include the level of tissue organization, the number of embryonic germ layers, the type of body symmetry, and the presence and type of body cavity. ### 1. Classification Based on Tissue Organization Animals are first divided into two subkingdoms based on whether they possess true, organized tissues. - **Subkingdom Parazoa** - The simplest multicellular animals. - They **lack true tissue organization**, though they exhibit a division of labor among their cells. - Typically asymmetrical. - Example: All sponges (Phylum Porifera). - **Subkingdom Eumetazoa** - Animals with true tissues. - Similar cells are grouped into coordinated units called **tissues**. - Tissues are assembled into larger functional units called **organs**. - Organs operate together as **organ systems**. ### 2. Classification Based on Number of Tissue Layers (Germ Layers) Eumetazoan animals are further classified by the number of embryonic germ layers they develop during . - **Diploblastic Animals** - Possess two germ layers: an outer **ectoderm** and an inner **endoderm**. - A non-cellular, jelly-like layer called **mesoglea** lies between the ectoderm and endoderm. - Exhibit a tissue level of organization but lack specialized organs. - Possess a simple nervous system (neuron net). - Have a single **gastrovascular cavity** with one opening that serves as both mouth and anus. - Example: Animals of Phylum Cnidaria (e.g., jellyfish, corals). - **Triploblastic Animals** - Possess three germ layers: an outer **ectoderm**, a middle **mesoderm**, and an inner **endoderm**. - The development of the mesoderm allows for the formation of complex organs and organ systems. | Feature | Diploblastic | Triploblastic | | :--- | :--- | :--- | | **Germ Layers** | Two: Ectoderm, Endoderm | Three: Ectoderm, Mesoderm, Endoderm | | **Middle Layer** | Mesoglea (non-cellular) | Mesoderm (cellular) | | **Organization Level** | Tissue level | Organ and Organ-system level | | **Example Phyla** | Cnidaria | Most other animal phyla (e.g., Platyhelminthes to Chordata) | ### 3. Classification Based on Body Symmetry The subkingdom Eumetazoa is divided into two grades based on body symmetry. #### Grade Radiata - Exhibit **radial symmetry**, where body parts are arranged around a central axis, like spokes on a wheel. - They have a distinct top and bottom but no defined right or left sides. - This symmetry is considered an adaptation for a *sessile* (stationary) lifestyle, allowing them to respond to stimuli from all directions. - Examples: Jellyfish, sea anemones. #### Grade Bilateria - Exhibit **bilateral symmetry**, where the body can be divided by a single plane (the midline) into two mirror-image halves (right and left). - This body plan is associated with cephalization (development of a head). - Possess distinct **anterior** (front/head), **posterior** (rear/tail), **dorsal** (top/back), and **ventral** (bottom/belly) surfaces. - This symmetry is considered an adaptation for *motility* (active movement). - All animals in this grade are triploblastic. - Note: Echinoderms (e.g., starfish) have bilateral symmetry in their larval stages but develop secondary radial symmetry as adults. | Feature | Radial Symmetry (Grade Radiata) | Bilateral Symmetry (Grade Bilateria) | | :--- | :--- | :--- | | **Definition** | Body parts arranged around a central axis. | Body divisible into two mirror-image halves along a midline. | | **Body Surfaces** | Top and bottom | Anterior, posterior, dorsal, and ventral | | **Lifestyle Adaptation** | Sessile (stationary) | Motile (active movement) | | **Germ Layers** | Typically Diploblastic | Triploblastic | | **Examples** | Jellyfish, Sea Anemone | Humans, Insects, Worms | ### 4. Classification Based on Body Cavity (Coelom) Animals are also classified based on the presence and type of a fluid-filled body cavity called a coelom, which is derived from the mesoderm. This leads to classifications such as *acoelomates*, *pseudocoelomates*, and *coelomates*. For detailed study of these groups, refer to . #### Protostomes vs Deuterostomes During embryonic development, triploblastic coelomates are further divided based on their cleavage patterns and the fate of the blastopore.