Characteristics of Animal Kingdom

Early stages of animal development, such as gastrulation, are the foundational characteristics that characterize animal phyla. In evolution, these initial processes seldom alter and frequently pave the way for subsequent developmental processes.

• Embryonic tissue layer- Three layers of embryonic tissue formed during gastrulation later differentiate to generate all the different types of cells in a mature animal. This kind of triploblastic growth is evident in all animals except the outliers: the phylum Porifera and phylum Cnidaria which are diploblastic.
• Symmetry- Bilaterally symmetrical creatures have their fronts and back distinctly different, while left and right sides are roughly mirrored images of one another. Example: Humans are bilaterally symmetrical. Radial symmetry is the architectural body plan where species are more or less spherical and have no left or right sides, and can move in either direction with equal ease. These include Cnidarians. Likewise, sponges don’t have any specific symmetry i.e., asymmetry.
• Cephalization- Cephalization is the state of presence of the head in organisms. It is lacking in cnidarians such as jellyfish. Most of our sensory organs, including the brains of humans and many other animals, are housed inside our heads. All bilaterally symmetrical animals experience cephalization.
• Body cavity- The digestive system and other internal organs develop in the bodily cavity.  Two huge cavities in our body include the thoracic cavity(which houses the lungs) and the abdominal cavity (which houses the digestive system). Animals are divided into three cavities based on the presence or absence of internal organs:
  • Coelomate: Many internal organs develop inside a cavity (the coelom) completely walled with mesodermal tissue in the body. Arthropods and chordates are coelomates.
  • Pseudocoelomate: The internal organs develop inside the coelom, partially walled with mesodermal tissue. Worms are pseudocoelomates.
  • Acoelomate: These organisms lack cavities and have a simple body plan.
• Segmentation- The bodies of many animals are segmented, which is evident in phylum Annelida, phylum Arthropoda, and phylum Chordata. Conversely, phyla Cnidaria are unsegmented.
• Digestive tract- All animal phyla- aside from sponges, have a gut or digestive system. Flatworms and cnidaria have digestive tracts with only one opening. Organisms with a complete digestive tract initiate from the mouth and end in the anus.
• Circulatory system- Some organisms have a closed system, while others have an open circulatory system.
• Exoskeleton- Arthropods have a hard exterior exoskeleton to defend their bodies, and through the process of molting, they shed their exoskeleton, which is termed ecdysis. Similarly, nematodes molt to grow, but they are more flexible exoskeletons, i.e., cuticles.