PHYLUM ECHIURA NEWBY, 1940 - ECHIURANS ... - IBSS Repository

PHYLUM ECHIURA NEWBY, 1940 - ECHIURANS 

General characteristics 

Morphology. The phylum Echiura consists of a group of unsegmented, bilaterally 

symmetrical coelomate animals, often mentioned in English literature as spoon-worms. The 

echiuran body, or trunk, is sub-cylindrical or sac-like in shape and muscular in composition. A 

muscular, sensory proboscis that is usually capable of great extension is nearly always present and 

is attached to the anterior region of the trunk. The proboscis is commonly bifid at the tip and cannot 

be retracted within the body of the animal, thus differing from the introvert of a sipunculan. The 

ciliated epithelium of the ventral surface of the proboscis functions to carry food particles to the 

mouth. The mouth is situated anteriorly at the base of the proboscis, and the anus posteriorly at the 

extremity of the trunk. The superficial covering of the trunk is cuticular epithelium with or withour 

papillae, in many echiurans containing a special toxic green pigment (bonelliin). A pair of setae is 

usually present on the ventral surface of the body just posterior to the mouth. These setae can 

elongate significantly when muscles are contracted, helping the animal in locomotion and in 

burrowing into the bottom deposits. Most species of the family Bonelliidae do not have ventral 

setae or have only one seta, but in the genera Acanthobonellia and Acanthohamingia these are 

numerous. One or two rings of anal setae encircle the posterior region in some genera. The body 

wall consists of longitudinal, circular, and oblique muscles. The longitudinal muscles may or may 

not be grouped into bands. The spacious body cavity or coelom, limited by the peritoneal 

epithelium, contains an elongated and much coiled alimentary canal, consisting of (1) a foregut, 

which is subdivisible into a pharynx, oesophagus, gizzard, and a stomach or crop, (2) a mid-gut or 

intestine proper, and (3) a hind gut or cloaca. A tube-like collateral intestine or siphon, the role of 

which in the echiuran physiology is still unclear, is associated with much of the mid-gut, its anterior 

and posterior ends joining the alimentary tract. 

One to four hundred nephridia lie in the body cavity; they are usually present in pairs and 

modified for use as gonoducts. The nephridia are attached to the ventral surface of the body wall 

with thin mesenteries. The external opening of a nephridium, commonly situated at the base of a 

ventral seta, is a nephridiopore, and the coelomic or internal opening a nephrostome. The 

nephrostomes may be placed basally or distally; their position is an important taxonomic character. 

Each nephrostome has a short stalk and is terminated in the fimbriate lips. In adult echiurans eggs 

and sperms are stored in the nephridia and discharged to the exterior through the nephridiopores. 

Before spawning the nephridia enlarge greatly, as they are filled with gametes. The section of the 

nephridium with gametes is called a storage organ. In some echiurans of the family Bonelliidae the 

anterior region of the nephridium is modified into a special chamber (androecium or male sac), in 

which males live. Pilger (1993) has determined that the nephridia in the echiurans have excretory 

function, too. But main organ of excretion is a pair of anal vesicles – hindgut sacs, which lie in the 

body cavity and discharge their products into the cloaca. Their surface usually bears numerous 

ciliated funnels connecting the coelom with the cloaca. The number and shape of the anal vesicles 

is an important taxonomic character. They may be sac-like, thread-like or branch simply or 

complexly. 

A closed vascular system consists of a dorsal, a ventral, and a neurointestinal vessel in the 

trunk and a median and two lateral vessels in the proboscis. The neurointestinal vessel may connect 

to the dorsal blood vessel through a ring vessel near the oesophagus. The blood is oxygenated 

chiefly in the proboscis. There is no blood vascular system in the family Urechidae; their hindgut 

functions as a respiratory organ, and the coelom as a vascular system. The central nervous system is 

a nerve cord that runs on the ventral side of the body and anteriorly expands into a peripharyngeal 

ring. The gonads are diffuse and lie in a mesentery above the nerve cord or in mesenteries near the 

cloaca. The sexes are separate; in the families Echiuridae and Urechidae they are indistinguishable 

externally, but in another (Bonelliidae) there is marked sexual dimorphism: the males are relatively 

small (1-3 mm) and carried on or in the females (commonly inside the nephridia, occasionally in 

the coelom, oesophagus, pharynx, or on the proboscis). There may be up to 20 males in a female. 

Males in some species of the Bonelliidae can reach a length of 20 mm (Metabonellia) or even 30-60 

mm (Acanthobonellia). In many deep-water echiurans males have not been found. Fertilization is 

usually external, but in the Bonellia eggs are fertilized inside the nephridia. Cleavage is spiral, and

the larva is a trochophore. The metamorphosis and settlement of the echiuran larvae have been 

thoroughly studied by many scientists (Hatschek, 1880; Baltzer, 1928; Newby, 1940; 1946). It was 

shown for the larval development in the family Bonelliidae (using Bonellia viridis as an example) 

that sex is genetically determined only in very small number of larvae, while others metamorphose 

into females after settling on the bottom, and males after settling on a female’s body (Jaccarini et 

al., 1983). 

Systematic position. Zoologists in the middle of the 19th century considered that the 

echiurans together with sipunculans and priapulids constituted a special group of animals which 

Quatrefages (1847) called the Gephyrea (from γεφυρα (Greek), a bridge). They had the impression 

that these groups of animals formed a link between the annelids and the echinoderms. Sedgwick 

(1898) raised the Sipunculoidea and Priapuloidea to the rank of phyla, but continued to consider the 

Echiuroidea as a class of the Annelida. The Echiuroidea was established as a phylum largely as a 

result of the studies of Newby (1940), who showed that annelids and echiurans differ considerably 

in their mode of development, that the mesodermal bands of developing echiurans show no trace of 

segmentation. The phyletic status of the group is now generally accepted (Zoological Records, 

Vermes Section; Fisher, 1946; Stephen & Edmonds, 1972; Meglitsch & Schram, 1991; Rouse & 

Fauchald, 1995, etc.). Zenkevich (1940) had an opinion, which was later supported by Ax (2000) 

that echiurans definitely are annelids, and the absence of segmentation in adult forms is just a 

secondary feature. This point of view was recently confirmed by evidence that echiuran larvae have 

metamerically disposed units of nervous system which may correspond with annelid’s segments, 

witnessing that echiurans originate from segmented animals (Hessling & Westheide 1999; 

Hessling, 2002, 2003). Molecular analysis also indicates that echiurans are related to annelids, and 

some scientists consider them as a separate group within the phylum Annelida (Staton, 2003), while 

others believe that they are close to the polychaetes of the family Capitellidae (Bleidorn et al., 

2003; Hall et al., 2004) or Trichobranchidae (Colgan et al., 2006). 

Ecology. All echiurans are marine, except for a few species that occur in brackish water. 

Their bathymetric range is extensive, from the intertidal zone to a depth of ten thousand meters. 

They are soft-bodied, almost defenseless creatures that live in burrows. Some echiurans prefer 

burrows which are already dug, others dig them themselves; there are species that make U-shaped 

burrows, having two openings. These burrows in sand or mud often attract many commensals: 

crustaceans, polychaetes, mollusks, sipunculans, etc. (Anker et al., 2005). In most echiurans 

locomotion is much restricted, but there are forms which can crawl out of their burrows and move 

by peristaltic contractions of the muscles. Some members of the family Bonelliidae use their 

proboscis for locomotion. 

Most echiurans are typical deposit-feeders. The spoon-like proboscis aids the species of the 

family Bonelliidae to direct sand, mud, detritus, etc. from their surroundings into their mouths and 

extract organic matter from the ingested material. The deep-water echiuran Kurchatovus 

tridentatus, found in sunken pieces of wood, roots of sea grasses, and shells of cocoa-nuts, is likely 

to be a xylophage, and members of the genus Urechis (family Urechidae) feed on seston. They 

secrete a slime net from the glands at the base of the proboscis and pump water together with 

suspended particles through this net by peristalsis. Intermittently, the net becomes clogged with 

trapped food, is consumed, and a new net is constructed. 

Echiurans have definite practical value: they serve as food for rays and flatfish off the coast 

of Chile (Gay, 1854), and for pinnipeds in the Russian Far East seas (Zaks, 1933). Some species are 

used as bait: Urechis unicinctus in Japan and Korea (Fisher, 1952), and Echiurus echiurus in 

Belgium (Skorikov, 1909). Koreans also catch U. unicinctus with iron hooks, dry them and eat 

(Sato, 1939). 

Collection, fixation, and identification 

Echiurans must be treated with care on collection, because their proboscis is deciduous and 

usually breaks off entirely or partially. Bottom samples quite often contain only echiuran proboscis, 

which is absolutely insufficient for normal identification. Live animals must be narcotized by 

placing them in a 7% solution of magnesium chloride or by placing them in cool sea water to which 

is added very carefully a small quantity of ethyl alcohol or some crystals of menthol. It is highly 

desirable to relax or narcotize a specimen before it is fixed and to leave it in the relaxing agent for

up to 24 hours in order to prevent it from becoming contracted and its internals from becoming 

distorted when it is directly plunged into fixatives. Echiurans should be fixed in a 4% solution of 

formalin in sea water, but later animals have to be placed in 70% alcohol for storage. It is better to 

take 6-8% formalin for the fixation of large echiurans, otherwise some of their internals may 

become macerated. 

In order to identify an echiuran it is nearly always necessary to dissect and examine its 

internal morphology. The animal is placed with its ventral side down in a dish, and the dissection is 

performed by carefully cutting the body wall longitudinally along the mid-dorsal line of the trunk 

with surgical scissors. If there are any signs of maceration, especially of the thin tissues of the mudfilled 

intestine, it is often possible to wash away some of the macerated material by directing on to 

it a fine stream of water from a pipette. The nephridia of the specimens belonging to the family 

Bonelliidae may contain dwarf males, morphological features of which (presence or absence of 

ventral setae) are sometimes used in systematics. 

Main references: Fisher, 1946; Stephen & Edmonds, 1972; Murina, 1984, 1993; Pergament, 

1961; Nishikawa, 2002. 

Systematic part 

About 160 species of the Echiura are described to date. Most of them belong to the families 

Echiuridae (more than 80 species) and Bonellidae (about 70 species) (Murina, 1993). The family 

Urechidae consists of only 4 species. The present paper accepts a term ‘echiuran’ or ‘spoon-worm’ 

in reference to an animal belonging to the phylum and a term ‘echiurid’ in reference to the member 

of the family Echiuridae, as was proposed by Stephen and Edmonds (1972). 

The same authors (1972) divided the phylum Echiura into three orders (Echiuroinea, 

Xenopneusta, and Heteromyota) and four families (Echiuridae, Bonelliidae, Urechidae, and 

Ikedaidae). Some authors (DattaGupta, 1976; Saiz-Salinas, 1987) separated the family 

Thalassematidae from the family Echiuridae. Stephen and Edmonds regard it as a subfamily, which 

is accepted in the present paper. Nishikawa (2002) has recently studied the internal morphology of 

Ikeda taneioides, cancelled the family Ikedidae and the order Heteromyota. The present phylum 

Echiura includes two orders and three families. 

KEY TO THE ORDERS OF THE PHYLUM ECHIURA 

1 (2). Posterior region of intestine enlarged and acts as respiratory organ; cloaca functions as pump. 

Proboscis short, reduced to slightly pointed conical lobe, never breaking off during fixation. 

Ventral and anal setae present ……………………………………. Xenopneusta 

2 (1). Posterior region of intestine not enlarged and does not act as respiratory organ; cloaca not 

modified. Proboscis normally developed, may break off during fixation. Ventral and anal 

setae present or absent ………………………………………….. Echiuroinea 

Order Echiuroinea Bock, 1942 

Proboscis usually normally developed, bifid or not bifid at tip. Blood vascular system 

present. Posterior region of intestine not enlarged, does not act as respiratory organ; cloaca not 

modified, but may be expanded. Nephridia paired or unpaired, varying in number. Ventral and anal 

setae present or absent. 

KEY TO THE FAMILIES OF THE ORDER ECHIUROINEA 

1 (2). Species with marked sexual dimorphism. Females with bifid proboscis, occasionally with 

simple proboscis. Nephridia commonly unpaired (species from Sea of Japan with one 

nephridium). Anal setae absent; ventral setae present or absent (species from Sea of Japan 

without ventral setae). Anal vesicles usually branched …………. Bonellidae (p. ) 

2 (1). Species without sexual dimorphism. Proboscis not bifid. Nephridia paired, or unpaired and 

numerous (genus Ikeda). One pair of ventral setae present; anal setae absent or form two 

rings. Anal vesicles sac-like and not branched ……………….. Echiuridae (p. )

Family Bonelliidae Lacaze-Duthiers, 1858 

Echiurans with marked sexual dimorphism. Males small, 1–3 mm, occasionally to 60 mm 

long, without proboscis, with substantially reduced internal organs, sometimes with ventral setae; 

usually found in female nephridium. Females with long bifid proboscis (in some species proboscis 

not bifid). Ventral setae usually either two or absent, occasionally one or numerous. Anal setae 

absent. Muscles not grouped into bands. Usually one or two, sometimes three nephridia present. 

Anal vesicles usually branched, or not branched, thread-like (genera Zenkevitchiola, 

Choanostomellia, Pseudoikedella). Neurointestinal and dorsal blood vessels usually connected 

through blood capillaries or lacunae of intestine. 

The Key includes four genera; three genera and three species have been recorded in the 

Russian waters of the Sea of Japan. 

KEY TO THE GENERA OF THE FAMILY BONELLIIDAE 

1 (2). Anal vesicles falciform with short tree-like branches. Proboscis somewhat bifid at tip 

………………..……………………………..………………….Nellobia 

2 (1). Anal vesicles branched. Proboscis markedly bifid. 

3 (6). Nephrostome located distally. Anterior part of nephridium not modified into androecium. 

4 (5). Nephrostome placed on long stalk; nephridium not reaching posterior end of body. Proboscis 

shorter than body. Males small ……………………………………….Eubonellia 

5 (4). Nephrostome placed on short stalk; nephridium long, almost reaching posterior end of body. 

Proboscis longer than body ……………………….……………….Ikedella 

6 (3). Nephrostome located almost basally. Anterior part of nephridium not modified into 

androecium. Males relatively large. (20–35 mm long) ……………..Achaetobonellia 

Family Echiuridae Quetrefages, 1847 

Echiurans without sexual dimorphism. Proboscis usually well-developed, but never bifid. 

Pair of ventral setae present in all genera, while anal setae present only in Echiurus. Longitudinal 

muscles continous, in some genera of subfamily Thalassematinae forms longitudinal muscle bands. 

Nephridia usually paired; one to seven pairs (sometimes to 10-32 pairs) present; more rarely 

nephridia unpaired and very numerous, to 200-400 (genus Ikeda). Anal vesicles usually elongate or 

swollen sacs and not branched. Dorsal and neurointestinal blood vessels usually connected through 

ring vessel in posterior region of foregut. 

Remarks. Nishikawa (1998) showed that Ikeda taneioides has typical for the echiurids 

disposition of muscle bands in the body wall, which became sufficient grounds to consider the 

family Ikedidae (=Ikediidae) to be a junior synonym of the Echiuridae. 

KEY TO THE SUBFAMILIES OF THE FAMILY ECHIURIDAE 

1(2). Two rings of setae surround posterior (anal) region of body. Post-pharingeal diaphragm, 

incompletely separates body cavity into two parts. Nephrostome without spirally coiled lips 

……………………………………………………………..Echiurinae 

2(1). No rings of setae surround posterior (anal) region of body. Post-pharingeal diaphragm absent. 

Nephrostome may or may not possess spirally coiled lips .……… Thalassematinae 

Subfamily Thalassematinae Forbes et Goodsir, 1841 

Anal setae absent. Longitudinal layers of muscles continuous or grouped in bands. Postpharyngeal 

diaphragm absent. Nephrostome lips may or may not be spirally coiled.

The subfamily includes eight genera. In the Russian waters of the Sea of Japan one member 

of one genera has been found and one more genus may be found. 

KEY TO THE GENERA OF THE SUBFAMILY THALASSEMATINAE 

1(2). Nephrostomal lips large, pronounced, expanded into leaf-shaped structure; anterior extremity 

of proboscis expanded or fan-shaped…………………………………Arhynchite. 

2(1). Nephrostomal lips small, poorly developed, not expanded into leaf-shaped structure; anterior 

extremity of proboscis rounded and not expanded or fan-shaped………….……………. 

……………………………………………………………………………Thalassema. 

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