ABSTRACT
The hemichordates are a relatively small taxon of invertebrate deuterostomes. With ~ 130 species, they comprise the solitary vermiform enteropneustic acorn worms and sessile colonial pterobranchs. Pterobranchs include cephalodiscids and rahpdopleurids; both of them are heterogonic, reproduce sexually and asexually. Being colonials, both cephalodiscids and rhabdopleurids generate eggs and sperm from the constituent female and male zooids. With the largest eggs, the harrimaniids display the simplest direct life cycle. The ptychodorids generate more number of relatively small eggs and pass through tornaria larval stage. A series of paired gonads are located between post-branchial and prehepatic trunk of enteropneusts. The experiments of Tweedell have demonstrated that unlike the bidirectional regeneration of echinoderms, regeneration in hemichordates is unidirectional, only one half regenerates. The regenerative potential of enteropneusts is very low and not comparable to that of echinoderms. Incidentally, there are claims on the possible bidirectional asexual reproduction in a few enteropneust species.
