Question

discuss at least five major trends in the pattern of invertebrate evolution, indicating why these changes were important.

Answer 1

Trends in invertebrate evolution are multicellularity, tissues and organs,radial and bilateral symmetry,cephalization,cephoderm, complete digestive syatem,coelem, segmented body and notochord.

evolution of tissues,it was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called endoderm. The two cell layers allowed different types of tissues to form.

Another trait that evolved early on was symmetry. A symmetrical organism, in contrast, can be divided into two identical halves. The coral pulp has radial symmetry.it was the first symmetry involved.The coral has a distinct top and bottom but not distinct ends. It can be divided into identical halves like a pie, but not into right and left halves. Animals with radial symmetry have no sense of directions such as forward and backward or left and right. This makes controlled movement in these directions impossible.

Flatworms represent the next stage of invertebrate evolution. They evolved cephalization. This is the concentration of nerve tissue at one end of the body, forming a head region. This is highly adaptive. It allows central control of the entire organism. Cephalization was first step in the evolution of a brain.

An outcome of cephalization was bilateral symmetry.For beetle With concentrated nerve tissue at the head but not at the tail end, the two ends of the body are distinct from each other. The animal can be divided down the middle to form identical right and left halves. It allows the animal to tell front from back and left from right. This is needed for controlled movements in these directions.

Ancestors of flatworms also evolved mesoderm. This is a third layer of cells between the ectoderm and the endoderm. Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle.