The journey towards a chordate life

Phylum Hemichordata- 

This phyla consists of a few hundred species (identified as of now). They are very useful subjects for the study of vertebrate evolution. The class Graptolita consists of fossils, which are most useful in this area of study.

The term 'Hemichordates' mean 'half chordates'. They show the chordate characters of a notochord in the anterior portion of their body. a dorsal nerve cord and pharyngeal gill slits or branchial openings. They are characterized by a three-fold division of their body. They belong to three groups the Enteropneusts (burrowing acorn worms), the Pterobranchs (the colonial hemichordates) and the graptolites (the fossil group belonging to the Silurian and Ordovician periods).

Class Enteropneusta

Class Pterobranchia

Rhabdopleura sp

Class Graptolita

Didymograptus sp

Source: http://www.ucmp.berkeley.edu/chordata/hemichordata.html