<P> A plexus of nerves lies underneath the skin, and is concentrated into both dorsal and ventral nerve cords . While the ventral cord runs only as far as the collar, the dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region . This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates . In acorn worms, it seems to be primarily involved with coordinating muscular action of the body during burrowing and crawling . </P> <P> Acorn worms have no eyes, ears or other special sense organs, except for the ciliary organ in front of the mouth, which appears to be involved in filter feeding and perhaps taste (3). There are, however, numerous nerve endings throughout the skin . </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> <P> One theory is that the three - part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes . Studies have shown that the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system, they are controlling the development of the different body regions . </P>

Acorn worms and pterobranchs have a body plan consisting of a