[ad_1]
Animal embryos develop from blastulas into germ layers, with triploblastic animals having three layers and diploblastic animals having two. Diploblastic animals have radial symmetry and are simple in structure, while triploblastic animals have bilateral symmetry and develop more complex internal organs. Triploblasts can be further divided by body cavities. The two main types of diploblastic animals are Cnidaria and Ctenophora. Animal life evolved from single-celled ancestors, with triploblasts becoming numerous during the Cambrian explosion. Sponges are an exception to this classification system.
As an animal embryo begins to form, it develops a round, hollow group of cells called a blastula. These cells then begin to differentiate into distinct layers known as germ layers, which will eventually develop into different groups of organs and body parts. Most animals develop from blastules that have three germ layers: an outermost layer called the ectoderm, a middle section known as the mesoderm, and an inner layer called the endoderm. These animals are known as triploblastic. Some more primitive animals, such as jellyfish, have blastules with only two layers, the ectoderm and the endoderm; these are diploblastic.
Structure and development of the body
Diploblastic animals have radial symmetry, which means they can be divided into two similar halves in many different ways, while triploblastic animals have bilateral symmetry, which means that there is only one way to divide them into similar halves. As a general simplification, it can be said that the ectoderm develops into the outer skin and the endoderm ultimately forms the digestive system, while the mesoderm – present only in triploblastic animals – develops into muscles and various internal organs. Diploblastic organisms are therefore very simple in that they essentially have only an outer skin, which may include a rudimentary nervous system and digestive tract. In more complex triploblastic animals, such as mammals, things are more complicated. The brain, for example, develops from the ectoderm, along with the rest of the nervous system; some of the internal organs, such as the liver, pancreas, and various glands, are derived from the endoderm, along with the digestive system.
Triploblasts can be further divided in terms of body cavities. The simplest types, such as flatworms, have no cavity other than the digestive tract. Some other animals have a fluid-filled space between the digestive tract and the mesoderm. The most advanced animals have a cavity that lies entirely within the mesoderm. This allows them to push food through the intestines by muscle contractions.
The two main types of diploblastic animals are Cnidaria and Ctenophora. Cnidarians are mostly marine, but there are some freshwater members of the group, which includes jellyfish, corals, sea pens, sea anemones, sea pansies, sea wasps, and gorgonians. Ctenophores are a separate marine group, sometimes referred to as comb jellies. These simple animals lack true organ systems, but they do have a cavity in which food is digested, and they may have nerves, sensory systems, and reproductive parts.
Animal evolution
Animal life is generally thought to have evolved from a single-celled ancestor, through diploblastic multicellular organisms, to more complex triploblastic life forms. It is possible, however, that the two types of animals arose independently from different unicellular ancestors, or even that diploblasts evolved from triploblasts and became simpler in structure. These issues are an ongoing area of research, but it is clear that the two types of animals differentiated from each other at a very early stage in animal evolution. There is fossil evidence of triploblasts dating back to about 700 million years ago.
Due to the fact that diploblasts do not have skeletons or other hard body parts well preserved in rock, the fossil evidence is very limited. This makes it difficult to determine exactly what very early animal life looked like, and recent studies attempting to solve problems relating to the evolution of different types of early animals have tended to focus on the genetic evidence of living descendants. It is clear, however, that triploblasts suddenly became very numerous and diverse during a period known as the Cambrian explosion, between 570 and 530 million years ago.
While almost all animals can be divided into radial diploblasts and bilateral triploblasts, sponges, also known as porifera, are an exception. Their cells are not organized into tissues, although there are different types with different functions. They also lack radial or bilateral symmetry. Sponges are the simplest living animals and are thought to have broken away from tissue animals at a very early stage in evolution.
