Adult Female
The external morphology of Bonellia viridis is very simple, it consists of a cylindrical trunk and a non fully retractable, flat and forked proboscis. They also have papillae all over the body which are easily seen.

Bonellia viridis have a large coelomic space extending in the proboscis, lined by peritoneal tissue. They have a single ventral nerve cord running from the tip of the proboscis down to the anus. It runs parallel to the blood vessel which contains hemoglobin (Pechenik, 2010). Bonellia viridis nerve cord is metameric meaning it is segmented. In addition, paired peripheral nerves regularly arranged support the classification of Echiura as a taxon of Annelida (Hessling, 2003).  

Respiration is conducted by diffusion across the body (Pechenik, 2005) and oxygen is transported via hemoglobin to the muscles and tissues. 

 

Compared to most animals, they have a single nephridia (Stephen et al. 1972), located next to the pharynx, along the nerve cord and blood vessel. 

The digestive system is very long and coiled, in order to absorb all the necessary nutrients from all the wastes. 

Gonads are absent, gametes are produced by the peritoneal tissue lining the coelom (Pechenik, 2005). 

Bonellia viridis have anal sacs which are located in the posterior area of the coelom, next to the anus. These sacs open into the gut and are ramified, highly ciliated funnels opening to the coelom. Additionally, females use their modified metanephridium as oviducts which is open to the coelom and to the exterior (Harris et al. 1981).    

The male Bonellia viridis has a much simpler structure. It measures only between 1 and 3 mm and lacks a proboscis. The epidermis is ciliated and additionally to the ventral nerve cord, some nerves are found within the body wall. The gut consists of a single layer of epidermis and lacks an anus. A pair of nephridia is present, compared to the female which only has one. Males main constitution is the sperm sac. It can occupy up to 2/3 of the animal and is located on the ventral side. The sperm ducts are ciliated and embedded in the body wall (Schuchert et al. 1990).