Sipuncula
Marine worms with a retractable introvert and unsegmented body.
Sipuncula, also known as sipunculid worms or peanut worms, is a class of marine annelid worms comprising about 162 species. Once considered a separate phylum of unsegmented worms, they were reclassified as a class within Annelida based on molecular evidence, having secondarily lost their segmentation. These worms are found in oceans worldwide, primarily in shallow waters, where they burrow in sandy or muddy substrates or inhabit crevices and under stones.
Quick Facts
- Taxon
- Sipuncula
Facts from the source article.
Lore & Background
Sipunculans vary in size, with most species under 10 cm. Their body is divided into an unsegmented, bulbous trunk and a narrower, anterior introvert that can be fully retracted. The mouth is at the tip of the introvert, surrounded in most groups by a ring of short tentacles. With no hard parts, the body is flexible and mobile. When threatened, they contract into a shape resembling a peanut kernel, giving them the common name 'peanut worm'.
Reader's Guide
Sipuncula represents a group of marine annelids that have secondarily lost segmentation, making them a key example of evolutionary reversal in body plan. Their reclassification from a separate phylum to a class within Annelida, based on molecular work, highlights the importance of genetic analysis in resolving long-standing taxonomic debates. The group is relatively understudied, with an estimated 162 species worldwide. Sipunculans play ecological roles as deposit feeders and, in some cases, filter feeders, and they are used as food in some countries in southeast Asia. Their anatomy includes a unique introvert, a hydrostatic skeleton, and a coelomic system that lacks a vascular blood system, relying instead on coelomic fluid containing haemerythrin for gas exchange. The presence of anal and caudal shields in some genera aids in boring into rock. The group's phylogenetic history—from annelid classification to molluscan alliance and back to annelid—illustrates the challenges of classifying organisms based on morphology alone. Their significance lies in their contribution to understanding annelid evolution, larval development, and the plasticity of body segmentation.
Did You Know?
- Sipunculid worms are also called peanut worms because they can contract into a shape resembling a peanut kernel.
- The name Sipuncula derives from the Latin siphunculus, meaning 'little tube'.
- Most sipunculans are deposit feeders, extending their introvert to gather food particles.
- Sipunculans have a simple circulatory system with dorsal and ventral vessels; gas exchange is also aided by the coelom containing haemerythrin.
More in Worms And Worm-Like Invertebrates 1-23
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