85. Metazoa: important evolutionary changes associated with the transition to multicellularity. Phylum Cnidaria (Coelenterata). Genus Hydra. Genus Obelia.

Metazoa: important evolutionary changes associated with the transition to multicellularity. Phylum Cnidaria (Coelenterata). Genus Hydra. Genus Obelia.

Metazoa and the Transition to Multicellularity

• Metazoa refers to all multicellular animals and represents a major evolutionary leap from single-celled organisms.
• The transition to multicellularity involved several important structural and functional changes over time.

Key Steps in the Evolution Toward Multicellularity

• One of the earliest changes was the development of flagellated cells that could move in coordination. These cells began forming aggregates that swam together, using the synchronized beating of their posterior flagella. This adaptation is considered a fortunate evolutionary step.
• These early cell colonies eventually formed hollow spheres where cells were organized into a surface layer, marking the first attempt at forming a tissue-like structure.
• Inside this primitive layer, some cells began to specialize, such as for reproduction, setting the stage for cellular differentiation.
• Sponges, belonging to the phylum Porifera, are examples of animals that exhibit simple cell associations without true tissues.

The Radiata: Early Tissue-Forming Animals

• The first animals to develop primitive tissues were members of a group called the Radiata, which includes cnidarians.
• Radiata introduced several evolutionary innovations:
• Radial body symmetry, where body parts are arranged around a central axis
• A primary body axis, establishing a sense of directionality in body structure
• The emergence of the first true tissues known as ectoderm and endoderm, which are also called germ layers
• These two tissue layers form the basis of diploblastic animals and allow for basic body organization and division of labor among cells.
• Later in evolution, animals began to develop a third tissue layer, the mesoderm, creating triploblastic animals.
• The mesoderm enabled the formation of muscles, excretory systems, and circulatory systems, allowing for more advanced movement, internal regulation, and complexity.

Phylum Cnidaria (also called Coelenterata)

• Cnidarians are part of the Radiata and are considered true multicellular animals with organized tissues.
• They show radial symmetry and possess the first clear polarity or orientation in their body structure.
• Cnidarians have two distinct tissue layers:
• The ectoderm on the outside
• The endoderm on the inside
• These layers are separated by a gelatinous noncellular substance called mesoglea.
• They have a gastrovascular cavity, a central digestive compartment that allows extracellular digestion of food.
• Cnidarians are known for their specialized stinging cells, called cnidocytes, which contain structures that inject venom into prey. This makes them highly efficient predators.