• Bonellia is a marine and burrowing echinoid. It lies buried in the mud.

• It exhibits’ very interesting case of sexual dimorphism with giant female and dwarf male without segmentation. Body divisible into anterior proboscis and posterior trunk.

• Female is 5 cm long, ovoid in shape and green in colour due to the presence of a pigment called Bonellein.

• The proboscis is very much elongated forming long spatulate in front of the mouth.

• The mouth is found at the base of proboscis and the anus at opposite ends.

• Excretory organ are a pair of anal vesicle, one or two nephiridia which act as a gonoduct.

• There is a closed blood vascular system.

• Male is very small, ciliated and without proboscis.

• Fertilization is internal.