The phylum Hemichordata is a particularly interesting group of animals that apparently represent an intermediate group between the Echinodermata and the Chordata. This marine phylum contains three major groups; the acorn worms, the pterobranchs, and a major extinct group of animals called graptolites. The hemichordates share the same type of larva as the echinoderms and has have several molecular features in common. However the adult acorn worm also has a series of pharyngeal gill slits (the dark dots in the figure of the acorn worm) which are apparently homolgous with the pharyngeal structures found in the primitve chordates such as the tunicates and Amphioxus. The primitve pharyngeal gill slits originally served two functions; as a respiratory organ and to filter out organic matter for food. Water and organic material enters the mouth and is forced through the pharyngeal slits, filtering out the food which is passed to the digestive system. Hemichordates also have a dorsal, hollow nerve cord probably equivalent to the dorsal, hollow nerve cord partially characteristic of the chordates. Hemichordates to dot appear to have a notochord, the most diagnostic feature of the chordates.

The acorn worms are marine animals commonly living in the mud of tidal flats. They are easily recognized by the anterior tough, flexible proboscis (or snout). The proboscis is followed by a collar. Cilia on the proboscis gather organic matter, and the organic matter and water pass through the mouth and are filtered by the phryngeal slits. The dorsal nerve cord is usually restricted to the collar.

Pterobranchs are a much less common group. The group is marine and resemble, to a degree, a shortened, curved version of the acorn worm. However the proboscis is modified into a structure called a feathery "lobophore" that filters organic matter from the surrounding water. The pterobranchs are colonial colonies and life in tubes composed of chitin. The greatest interest of the pterobranchs is their relationship with a major extinct group of animals called graptolites. Graptolites are known from the Middle Cambrian to the Lower Mississippian. The graptolites were a major component of the early marine faunas and are important index fossils.

Acorn Worm - Balanoglossus

Pterobranch - Rhabdopleura

The cordates include three living classes of animals; the Urochordata, Cephalocordata, and the Vertebrata. The chordates have three major defining characteristics:

`1. A dorsal hollow nerve chord (the brain and spinal chord of humans).

2. Clefts in the wall of the throat region usually referred to as phryngeal gill slits (modified for other functions in terrestrial vertebrates) used for respiration and feeding in the more "primitive" chordates.

3. An original dorsal cartilaginous structure called a notochord.The notochord gives the animal structural support. The notochord is gradually replaced by the vertebrae as one moves higher up in the vertebrate phylogeny. However the notochord persists in the vertebrate embryo and is also still present in some fish and a few fossil amphibians and reptiles.

 Class Urochordata (Sea Squirts, ascidians, tunicates, etc.)

The Urochordata are known by a number of names of which the three listed above are possibly the most common. The adult tunicate would seem to have few things in common with vertebrates. The animal is basically a single large filtering mechanism. Water with organic matter is taken in through the mouth, the organic material is filtered out through the large, basket shaped pharyngeal structure, and the excess water ejected through the "atriopore". When distubed some species eject water forcefully through the mouth, hence the name sea squirt.

The relationship between the tunicates and the rest of the vertebrates is far more obvious in the larvae. The larval urochordate has a distinct notochord (the gray structure in the figure) and a distinct dorsal, hollow nerve chord (in black in the figure). The pharyngeal structure is also present, but much smaller than in the adult.

Urochordates are marine, sedentary animals and can be either solitary, or colonial.

 Class Cephalochordata (Amphioxus, Lancets)

There are about 30 species of these small, marine animals. Amphioxus (genus Branchiostoma) is capable of swimming, but spend most of its time in shallow water, buried tail down in the mud filtering organic material from the water. The general body plan is similar to a larval tunicate. However in no other group are the defining features of the chordates so well defined with a strong dorsal, hollow nerve cord, and notochord.

 Class Vertebrata

The vertebates (including us) are characterized by the development of a cartilaginous or bony vertebrate column to surround and protect the dorsal nerve chord. The earliest vertebrates were the Agnatha are represented by the living groups the Lampreys and Hag Fish, and the important extinct group the Ostracoderms. The Agnatha lack a jaw and mouth is simple. The ostracoderms were covered with bony plates and very fish like. They lived from the Silurian to the Devonian. The ostracoderms gave rise to the placoderms.

The placoderms are the first vertebrates with a jaw, an important innovation for hunting and feeding. The jaws are derived from cartilaginous support bars found in the phryngeal region of the primitive chordates. Later some of these phryngeal bars would also be incorporated as small bones in the ear of the more advanced vertebrates. The hindged jaw was apparently a very important advance in vertebrate evolution and the placoderms are believed to have given rise to the rest of the vertebrates.