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List the characteristics of the phylum Chordata. Compare and contrast the three subphyla of the chordates.

The characteristics are listed below together with diagrams
During their embryonic development, all chordates pass through a stage called the pharyngula [View] with these features:

- A dorsal, tubular nerve cord ("1") running from anterior to posterior. At its anterior end, it becomes enlarged to form the brain.

- A flexible, rod like notochord ("2") that runs dorsal to the digestive tract and provides internal support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before maturity.

- Pairs of gill pouches. These lateral out pocketings of the pharynx are matched on the exterior by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the exterior grooves, forming gill slits ("3"). These provide an exit for water taken in through the mouth and passed over the gills.

- A tail that extends behind the anus

There are three subdivisions of the chordates:

1. Cephalochordata
2. Tunicata: The cephalochordates and tunicates never develop a vertebral column. They are thus "invertebrates" and are discussed with the other invertebrates.
3. Craniata The vast majority of chordates have a skull enclosing their brain, eyes, inner ear, etc.). All but one group of these (the hagfishes) also convert their notochord into a vertebral column or backbone thus qualifying as vertebrates.

Vertebrata:

Although one group, the hagfishes, never replaces their notochord with a vertebral column, and thus might seem not to qualify as vertebrates, they share a number of other features with other vertebrates and certainly should be classified with them. Still uncertain is whether they represent the most primitive vertebrates or are simply degenerate vertebrates.

http://www.shsu.edu/~bio_mlt/Chap2.html
http://www.chordates.com/
http://en.wikipedia.org/wiki/Chordates
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vertebrates.html

2. Explain why the evolution of jaws around the mouth was a critical innovation in vertebrate evolution.

Because with true jaws the diet of the animal could be far more complex, i.e. the earliest fish were jawless and as such could only suck or filter feed and many are heavily armoured as a protection, with jaws the diet can be much more complex and you can defend yourself as well as kill and devour prey. These were startling innovations for the phylum to evolve.

http://serendip.brynmawr.edu/biology/b103/f97/projects97/Kagle.html
http://www.csupomona.edu/~dfhoyt/classes/zoo138/PRIM_FISH.HTML
http://www.nhm.ac.uk/nature-online/life/reptiles-amphibians-fish/fathomseminar-jaws/session3/no-fish-fathomseminar-jaws-session3.html

3. Although amphibians were the first animals to move onto dry land, they were still dependent on water for reproduction and lor respiration. What evolutionary adaptations allowed reptiles to become the first "true" land vertebrates?

The evolutionary adaptations are:

- The development of shelled eggs
- Internal fertilization
- Behavioural adaptations include some parental care
Developed efficient lungs and this allows independence of water because there skin ...