Phylum Echiura
Phylum Echiura
I. Historical Account
First described by Pallas in 1766 and placed in the genus Lumbricus.
In the mid nineteenth century, echiurans, sipunculans, and priapulids were placed in the class Gephyrea ("a bridge") in Annelida, because they were thought to link annelids with holothurians.
In 1898, Sedgwick raised Sipunculoidea and Priapuloidea to phylum rank but considered Echiuroidea to be a class of the Annelida.
Newby advocated phylum rank for Echiuroidea in 1940 on the basis of embryological studies.
Stephen proposed Echiura as an amended phylum name in 1964.
II. Definition of the phylum
Vermiform coelomate Bilateria usually possessing a non-retractible proboscis which is usually rolled ventrally, a trunk with a pair of setae set anterio-ventrally, and living in sand, mud, or in crevices. All but one genus have a closed blood-vascular system.
III. External features
Trunk is unsegmented, cylindrical in shape. The trunk may be smooth or bear slime-secreting papillae. there is one pair of anterior setae located ventrally just behind the mouth, except in Saccosoma, which lacks setae. Echiurus and Urechis have one or two rings of setae around the posterior end.
The proboscis is a cephalic lobe which usually rolls under ventrally on both sides. The epithelium of the proboscis secretes a cuticle except in the ventral ciliated grooves which transport food to the mouth, which opens at the base of the proboscis. (Saccosoma lacks a proboscis.) The proboscis may flare anteriorly or even bifurcate, as in Bonellia.
The anus opens at the posterior end.
Color is derived from pigment ceells in the subepidermal connective tissue. Echiurans may be brown, green, red, or transparent.
IV. Internal features
A. The body wall is like that found in sipunculans and annelids. The epithelium consists of simple columnar cells which secrete a cuticle. Subepidermal connective tissue contains pigment cells. There are underlying layers of circular, logitudinal, and oblique muscle fibers. Muscles may be arranged in sheets or bundles.
B. Coelom in trunk is wide, fluid-filled, and undivided by mesenteries. Coelomic fluid may contain sherical erythrocytes with hemoglobin and amoeboid leukocytes.
C. Digestive system
Consists of mouth, pharynx, esophagus, (stomach in some species), gizzard, intestine, and hindgut or rectum.
Mouth, pharynx, and esophagus are ectoderm lined with a sphincter at the posterior end of the esophagus.
VI Embryology and development
Sperm and egg membranes fuse on contact. A partial polyspermy block is effected as early as ten seconds after fertilization. The indented egg rounds out following fertilization.
Echiuran zygotes undergo spiral spiral cleavage. Gastrulation occurs at about the 148 cell stage by invagination of the vegetal plate.
Most echiurans have a free-swimming trochophore larva, except Bonellia whose larva are completely ciliated and crawl about on the substrate.
The free-swimming stage may last up to 70 days. Metamorphosis is gradual, and eventually the larva will settle out and take up adult life.
VII. Ecology and physiology
Marine and brackish worms.
Tropical to subtropical in distribution.
Benthic dwellers, from the intertidal zone to 9000 m.
Live in sand, mud, under rocks, and in coral crevics.
Most are detritus feeders, extending the proboscis over the surrounding substrate and passing food by ciliary action to the mouth.
In Urechis, the worm constructs a U-shaped tube, then constructs a mucus funnel which filters the seawater the worm pumps through the tunnel. The worm then collects the food-laden funnel and eats it,
Respiration occurs by diffusion.
Parastized by protozoans, platyhelminthes, nematodes, and annelids
Commensals include annelids, arthropods, and mollusks.
VIII. Classification
Classification in Echiura is undergoing revision
Dissection is necessary for identification.
Stephen and Edmonds classification includes 3 orders, 4 families, 34 genera, and 129 species.
Orders
Echiuroinea has a closed vascular system, posterior region of the rectum "non-respiratory"
Xenopneusta includes four species in the genus Urechis in which there is no closed vascular system, and in which the posterior portion of the rectum is thin-walled. Water is pumped in and out of this region.
Heteromyota includes echiurans with the longitudinal muscle layer lying outside the circular and oblique muscle layers. The nephridia are unpaired and numerous.
IX. Phylogeny
A. The structure of the body wall, the closed vascular system, and a transitory metamerism in embryonic development all indicate a close relationship to the annelids.
References
Barnes, R. 1980, Invertebrate Zoology. Philadelphia: Saunders Co. pp. 870-873.
Gardiner, M.S. 1972. The Biology of the Invertebrates. New York: McGraw-Hill, Inc. pp. 316, 426, 513.
Gould-Somero, M. 1975. "Echiura." in Giese and Pearse, Reproduction of Marine Invertebrates, Vol. III. New York: Academic Press. pp. 277-311.
Hickman, C.P. 1973. Biology of the Invertebrates, Second Edition, Saint Louis: The C.V. Mosby Co. pp. 311-315.
Marshall, A.J., and W.D. Williams. 1972. Textbook of Zoology Invertebrates, Seventh Edition. New York: American Elsevier Publishing Co., Inc. pp. 381-387.
Stephen, A.C., and S.J. Edmonds. 1972. The Phyla Sipuncula and Echiura. London: Trustees of the British Museum. pp. 343-508.
(This material comes from ZOO 6207, "Advanced Invertebrate Zoology", at the University of Florida, Spring Semester 1982, class taught by Dr. Frank Maturo. The text was scanned and OCR'd by Wesley R. Elsberry in 2006. Typographic errors are likely due to the scanning process. This particular presentation was prepared by Wesley R. Elsberry for ZOO 6207.)
