KINGDOM: Animals

BILATERALLY SYMMETRICAL ANIMALS

Bilaterally symmetrical animals differ from radially symmetrical animals because they have a front and back end as well as an upside and downside, or a top and a bottom.

It is speculated that the first ever bilaterally symmetrical animal is the urbilaterian although there is considerable debate about what this actually is.  It is reckoned that the urbilaterian existed on Earth more than 580 million years ago.

Bilaterally symmetrical animals are split into phyla.  Phyla being the next stage down in the life hierarchy to kingdoms.

Many phyla within the description of bilaterally symmetrical animals fit into the superphylum known as deuterostomia.

Deuterostomes are where you will find most commonly known animals, so if this is your interest then you may wish to skip the following digression which distinguishes between deuterostomes and other bilaterally symmetrical animals.

During the very early embryonic development of many animals, some of the cells alter to create an opening within the embryo called a blastopore.  In deuterostomia, the blastopore becomes the anus.

Animals within the superphylum of deuterostomia differ from protostomes where the blastopore becomes the mouth.

There are other phyla understood to be within the description of bilaterally symmetrical animals such as orthonectida, rhombozoa and chaetognatha.  It is suggested that acoelomorpha are also a phylum of bilaterally symmetrical animals, although this remains disputed.

Orthonectida are relatively unknown parasites of marine invertibrates.  Parasites are animals which seek to simply exist upon or within another animal.  Invertebrates are best described as animals other than fish, reptiles, amphibians, birds, and mammals.  In other words, invertebrates are animals with no spine, which in humans is referred to as the backbone.

Rhombozoa (also known as dicyemida) are also parasites which live in the kidneys of cephalopods.  Typical cephalopods are the squid, the octopus and the cuttlefish.

Chaetognatha, which are more commonly referred to as arrow worms, are marine worms which form a large part of what is described as plankton.

Acoelomorpha are a mysterious soft animal which is mainly marine.  Their status as a phylum outside the description of a protostome has been recently disputed.  The acoela are described as a flatworm with no gut which swim as plankton.

Humans are deuterostomes.