#nemerteans

Animal Classification

Excretory structures of nemerteans include

Nemertean excretion and osmoregulation rely on protonephridial system superficially similar to flatworm protonephridia but uniquely associated with circulatory vessels enabling filtration of blood. System comprises terminal flame bulbs with ultrafiltration weir leading to ciliated tubules converging to paired ducts opening via nephridiopores laterally in anterior region typically one to three pairs. Tubules intimately associated with lateral blood vessels facilitating ultrafiltration from blood into lumen driven by pressure. Number variable some highly branched others reduced. Unlike metanephr

Ref: Brusca & Brusca, Chapter 20: Nemertea protonephridia excretory system; NCBI

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Animal Classification

The proboscis of nemerteans lies in a cavity called

Defining autapomorphy of phylum Nemertea is presence of rhynchocoel separate fluid-filled coelomic chamber located dorsal to alimentary canal extending most body length not connected to digestive lumen, housing within anterior portion eversible proboscis apparatus isolated from gut. Proboscis remains invaginated and coiled within rhynchocoel when retracted attached posterior by retractor muscle to wall. During prey capture contraction of body wall and rhynchocoel circular muscles raises hydrostatic pressure rapidly everting proboscis through anterior rhynchopore above mouth. Rhynchocoel fluid

Ref: Brusca & Brusca, Chapter 20: Nemertea rhynchocoel proboscis; NCBI Nemertea

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Animal Classification

Nemerteans differ from flatworms because they have

Nemerteans superficially resemble turbellarian flatworms due to dorsoventrally flattened ciliated creeping body, soft unsegmented appearance, but fundamentally distinguished by several evolutionary advances: possession of true complete tubular system with separate anterior mouth and posterior anus allowing continuous unidirectional processing and fecal egestion, compared to incomplete blind branching gastrovascular cavity of Platyhelminthes, plus development of well-organized closed circulatory system with dorsal ventral vessels containing nucleated erythrocytes with hemoglobin absent in flatw

Ref: Ruppert et al., Invertebrate Zoology, Chapter 20: Nemertea complete gut closed circulation

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Animal Classification

Nemerteans are commonly known as

Nemertea comprise elongate unsegmented soft-bodied highly contractile predatory worms known as ribbon worms due to extremely long flattened ribbon-like body capable of enormous extension, some species like Lineus longissimus reaching over 30 meters longest animal known, often brightly colored. Dorsoventrally flattened marine benthic predominantly few freshwater terrestrial forms. Characterized by unique eversible muscular proboscis housed in separate rhynchocoel cavity, complete tubular gut and closed circulatory system. Active hunters employing proboscis often armed with stylet to spear polyc

Ref: Brusca & Brusca, Chapter 20: Nemertea ribbon worms; Campbell Biology Chapter 33

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