<P> Acorn worms are considered more highly specialised and advanced than other similarly shaped worm - like creatures . They have a circulatory system with a heart that also functions as a kidney . Acorn worms have gill - like structures that they use for breathing , similar to the gills of primitive fish . Therefore , acorn worms are sometimes said to be a link between classical invertebrates and vertebrates . Some also have a postanal tail which may be homologous to the post-anal tail of vertebrates . An interesting trait is that its three - section body plan is no longer present in the vertebrates , except for the anatomy of the frontal neural tube , later developed into a brain which is divided into three main parts . This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible . </P>
<P> Acorn worms are considered more highly specialised and advanced than other similarly shaped worm - like creatures . They have a circulatory system with a heart that also functions as a kidney . Acorn worms have gill - like structures that they use for breathing , similar to the gills of primitive fish . Therefore , acorn worms are sometimes said to be a link between classical invertebrates and vertebrates . Some also have a postanal tail which may be homologous to the post-anal tail of vertebrates . An interesting trait is that its three - section body plan is no longer present in the vertebrates , except for the anatomy of the frontal neural tube , later developed into a brain which is divided into three main parts . This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible . </P>
acorn worms and pterobranchs have a body plan consisting of a
acorn worms and pterobranchs have a body plan consisting of a
brain which is divided into three main parts
brain which is divided into three main parts