HiSET Science: Characteristics of the Major Animal Phyla

Characteristics of the Major Animal Phyla

Body Planes
Animals can exhibit bilateral symmetry, radial symmetry, or asymmetry. With bilateral symmetry, the organism can be cut in half along only one plane to produce two identical halves. Most animals, including all vertebrates such as mammals, birds, reptiles, amphibians, and fish, exhibit bilateral symmetry. Many invertebrates including arthropods and crustaceans also exhibit bilateral symmetry. With radial symmetry, the organism can be cut in half along several planes to produce two identical halves. Starfish, sea urchins, and jellyfish exhibit radial symmetry. With asymmetry, the organism exhibits no symmetry. Very few organisms in the animal phyla exhibit asymmetry, but a few species of sponges are asymmetrical.

Body Cavities
Animals can be grouped based on their types of body cavities. A coelom is a fluid-filled body cavity between the alimentary canal and the body wall. The three body plans based on the formation of the coelom are coelomates, pseudocoelomates, and acoelomates. Coelomates have a true coelom located within the mesoderm. Most animals including arthropods, mollusks, annelids, echinoderms, and chordates are coelomates. Pseudocoelomates have a body cavity called a pseudocoelom. Pseudocoeloms are not considered true coeloms. Pseudocoeloms are located between mesoderm and endoderm instead of actually in the mesoderm as in a true coelom. Pseudocoelomates include roundworms and rotifers. Acoelomates do not have body cavities.
Simple or primitive animals such as sponges, jellyfish, sea anemones, hydras, flatworms, and ribbon worms are acoelomates.

Modes of Reproduction
Animals can reproduce sexually or asexually. Most animals reproduce sexually. In sexual reproduction, males and females have different reproductive organs that produce gametes. Males have testes that produce sperm, and females have ovaries that produce eggs. During fertilization, a sperm cell unites with an egg cell, forming a zygote.

Fertilization can occur internally such as in most mammals and birds or externally such as aquatic animals such as fish and frogs. The zygote undergoes cell division, which develops into an embryo and eventually develops into an adult organism. Some embryos develop in eggs such as in fish, amphibians, reptiles, and birds. Some mammals are oviparous meaning that they lay eggs, but most are viviparous meaning they have a uterus in which the embryo develops. One particular type of mammal called marsupials give birth to an immature fetus that finishes development in a pouch. However, there are some animals reproduce asexually. For example, hydras reproduce by budding, and starfish and planarians can reproduce by fragmentation and regeneration.

Some fish, frogs, and insects can even reproduce by parthenogenesis, which is a type of self-reproduction without fertilization.

Modes of Temperature Regulation
Animals can be classified as either homeotherms or poikilotherms. Homeotherms, also called warm-blooded animals or endotherms, maintain a constant body temperature regardless of the temperature of the environment. Homeotherms such as mammals and birds have a high metabolic rate because a lot of energy is needed to maintain the constant temperature. Poikilotherms, also called cold-blooded animals or ectotherms, do not maintain a constant body temperature. Their body temperature fluctuates with the temperature of the environment. Poikilotherms such as arthropods, fish, amphibians, and reptiles have metabolic rates that fluctuate with their body temperature.