Echiurida

a phylum consisting of about 150 species of invertebrates. This small but highly distinctive group of sea worms was previously placed in the class Annelides. The cylindrical body ranges in length from 3 cm to 185 cm. The echiuroids are found in mud. They are equipped with a long (up to 1 m) nonretractable proboscis that is covered with cilia; these direct food particles to the mouth apparatus, which is located on the abdominal side of the body near the base of the proboscis. Behind the mouth are two abdominal bristles. The posterior end of the body is sometimes girdled by one or two rings of bristles. The broad secondary cavity has a tubular intestine terminating in an anus, as well as paired excretory organs or metanephridia and anal sacs equipped with ciliated funnels. The circulatory system is poorly developed. The nervous system consists of a ring of nerve tissue near the pharynx and an abdominal branch without broadenings of the ganglia.

The echiuroids are dioecious. A larva, or trochophore, hatches from the egg. Echiuroids of the genera Bonellia and Hamingia are characterized by sharply delineated sexual dimorphism. Dwarflike males with an extremely simplified structure are parasites in the excretory passages of the female; fertilization takes place during the passage of the egg through the excretory system.