Phylum SIPUNCULA

Sipuncula

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Phylum SIPUNCULA

Compiler and date details

30 May 2005 - Rick Johnson

Introduction

After Edmonds (2000)
The Sipuncula is a group of soft-bodied, unsegmented, coelomate, worm-like marine invertebrates. The body consists of a muscular trunk and an anteriorly placed, more slender introvert. A mouth is situated at the anterior extremity of the introvert and a long, recurved, spirally wound alimentary canal lies within a spacious body cavity or coelom. An anus lies dorsally, usually on the anterior surface of the trunk near the base of the introvert. Tentacles either surround or are associated with the mouth. Setae or bristles are absent. Two nephridia are present, occasionally only one. The nervous system, although unsegmented, is annelidan-like, consisting of a long ventral nerve cord and an anteriorly placed brain. The sexes are separate, fertilisation is external and cleavage of the zygote is spiral. The larva is a free-swimming trochophore. Sipunculans are also known commonly as 'peanut worms'.

Sipunculans live in burrows, tubes and protected places. The trunk may be stout and cylindrical, long and worm-like, globular, pear-shaped, flask-like or twisted into a spiral, and is larger than the introvert. The trunk wall is often thick and highly muscular. The introvert arises at or near the anterior extremity of the trunk and its diameter is noticeably less than that of the trunk. A highly elastic organ, the introvert is capable of considerable extension and at other times complete or partial retraction within the body cavity. Papillae and glandular and sensory bodies are usually present on the surface of the trunk and introvert; sometimes they are prominent. Hooks and/or spines that are often found on the surface of the introvert assist in food collection. A number of tentacles are associated with the mouth and are involved in both the exchange of respiratory gases and the gathering of food particles. The trunk encloses a large, fluid-filled coelom. Within the coelom a number of retractor muscles (one to four) connect the anterior extremity of the introvert to the wall of the trunk, usually to the mid- or posterior region of the latter. A very long alimentary canal lies more or less freely in the coelomic fluid; it comprises: (1) an oesophagus, attached for most of its length by mesenteries to the retractor muscles; (2) the descending and the ascending, spirally wound loops of the intestine, and; (3) a short straight rectum. Attached to the oesophagus is a tubular contractile ('polian' or 'compensatory') vessel which contains fluid and blood cells.

The external openings or nephridiopores of the nephridia are situated ventro-lateral at the anterior of the trunk. The nephridia are involved in excretion and possibly osmoregulation. They also serve as gonoducts. The coelomic fluid contains nucleated blood cells, which carry the respiratory pigment haemerythrin, coelomocytes and developing eggs or sperm. The nerve cord is ventral and its anterior extremity is connected by two oesophageal connectives to a dorsal cerebral ganglion or brain. Sensory structures such as tactile organs, eyespots, chemoreceptors and a nuchal organ are present in many species. The gonads develop at the base of the ventral retractor muscles and the sexes are separate and difficult to distinguish unless eggs or sperms are present in the coelom. Gametes pass from the coelom to the nephridia through a ciliated nephrostome and from the nephridia to the exterior through a nephridiopore. Fertilisation takes place in the sea and the zygote develops into a free-swimming trochophore larva. In some species, the trochophore undergoes further metamorphosis to form a pelagosphaera larva before finally becoming an adult. No signs of segmentation have been observed in any of the developmental stages of the animal.

Sipunculans live in tropical, temperate and polar seas, from the intertidal regions of the shore to the floor of the oceans at depths of 6860 m (Murina 1964; Stephen & Edmonds 1972; Cutler 1977). Fisher (1952) considered that they will live in any protected place that provides access to reasonably clear water and food. According to Cutler (1994), the phylum contains about 150 species. Until recently, the most useful general accounts of the phylum were those of Hyman (1959) and Tetry (1959). Cutler's (1994) monograph updates these works and adds sections on their systematics and evolution.

Note various phylogenetic analyses that place Sipuncula with Annelida, finding that they form a clade with Amphinomidae as early offshoots (e.g. Parry et al. 2014; Lemer et al. 2015; Weigert et al. 2015). Until a firmer classification is determined, Annelida and Sipuncula are maintained in this daetbase as separate groups.