INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 2 - G.I. Shcherbak - 1996

PHYLUM ECHIURIDA

Echiurans are marine benthic worms. Most species inhabit tropical seas. Only a small number of species extend into polar latitudes, such as Echiurus echiurus, which is widespread along the entire Arctic coastline. They are found at various depths, from shallow waters down to 9 km. A total of about 150 species are known. Their body consists of a thickened trunk that extends at the anterior end into a narrower Structure—the proboscis. The cutaneous-muscular sac is well developed, and the body cavity is an unsegmented coelom. The intestine is very long and convoluted. The Mouth is located at the Base of the proboscis, and the anus is at the posterior end of the body. The excretory system in adults is represented by anal vesicles and nephromixia. The Circulatory system is closed. The overall structural plan of The Nervous system resembles that of polychaetes, but it lacks ganglia. Echiurans are dioecious. Germ Cells originate from the coelomic epithelium. Development proceeds via metamorphosis, with a trochophore larva emerging from the egg.

Echiurans were previously classified under the phylum Annelida. It was believed that they evolved from polychaetes that had lost their segmentation, including external segmental Organs, metameric coelomic sacs, and so forth. However, most zoologists now consider them to be primarily unsegmented coelomate animals. Features of their embryonic development, the absence of metamerism, and the lack of a special growth zone in the larva clearly distinguish echiurans from Annelids and provide sufficient grounds for classifying them as an independent animal phylum containing a single Class of the same name.