From tunicates to humans

Although not the largest phylum, Chordata contains the most familiar species, including humans. All chordates have several things in common that occur at some stage of development. They have pharyngeal slits, which are openings that connect the inside of the throat to the outside of the neck. These are often used as gills. Their main feature, what they are named after, is the notochord, which is a rod that supports the nerve cord. The nerve cord is also present in all species. This is a bundle of nerve fibers which connect the brain with the muscles and organs, and is through which messages from the brain are sent. A tail is also present, which extends past the anal opening. In most species these features disappear with age. For example, the pharyngeal slits are only present in the human fetus. There are approx 44 000 species in 3 subphylums:

1. Cephalochordata
This is a small, very unusual subphylum of creatures commonly called lancelets or amphioxus. These animals are fish-like in appearance, but are invertebrates with a notochord, and a nerve cord right above it. They lack bones, a brain, eyes, and most other organs associated with the brain. There are 25 species, and they do not seem to be placed in any class. However, some experts do not call this a subphylum and they place it in a class of the same name:

Cephalochordata (lancelets)

2. Tunicata (Urochordata)
This is a large subphylum of unusual invertebrates that do not look like anything much more than a strange underwater worm or mushroom. They start off life as tadpole-like larvae with notochords and all the rest. This stage lasts only a short time, after which they anchor to the seabed and live a sedentary life. They completely change shape at this point, and it is hard to believe that they are in the same phylum as humans. The adults lack the notochord but do keep the pharyngeal slits. They have a highly-developed internal structure, with a heart and other organs. Tunicates are named for their protective covering, known as a tunic. This tunic is made up of cellulose, which is very rare in animals. There are 2000 species in 4 classes:

Appendicularia or Larvacea (free swimming tunicates)
Ascidiaceae (sea squirts)
Sorberacea (benthic tunicates)
Thaliacea (salps)

3. Vertebrata
This is the largest subphylum with the more well-known animals, including humans, reptiles, fish, etc. Every animal with a backbone is present in this subphylum. The notochord is developed at an early age, and is replaced with vertebrate. All vertebrates have a skeleton of either bone or cartilage. Their brain is protected by a boney cranium, and consists of three parts. They all have well-developed hearts with 2-4 chambers and have a closed circulatory system. There are 41700 species in 8 classes:

Amphibia (frogs, salamanders)
Aves (birds)
Cephalaspidomorphi (Lampreys)
Chondrichthyes (cartilaginous fish)
Mammalia (mammals)
Myxini (Hagfish)
Osteichthyes (bony fish)
Reptilia (crocodiles, snakes, turtles)

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