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#gastropods

3 public questions tagged with this topic.

Animal Classification

Torsion results in the anus opening

Torsion fundamentally reorganizes gastropod body architecture during larval development. Rotation of visceropallium through 90 to 180 degrees brings originally posterior mantle cavity containing ctenidia, osphradia, anus and nephridiopores to anterior position dorsal to head and anterior to heart. Alimentary canal becomes sharply U-shaped, anus now discharging above mouth near head, creating sanitation problem. Advantage lies in ability to withdraw head-foot into shell with mantle cavity opening anteriorly facing incoming water current, enhancing respiratory flushing and predator protection, but causing fouling risk solved later by detorsion or shell slits.

Ref: Campbell Biology, 12th ed., Chapter 33: Gastropoda torsion consequences; Brusca & Brusca

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Animal Classification

Veliger larva is typical of

Veliger represents characteristic free-swimming planktotrophic larva of marine molluscs, directly derived from trochophore via development of expanded ciliated velum. It possesses two large ciliated lobes forming velum for swimming and particle capture, together with developing foot, mantle secreting protoconch, shell gland and visceral anlage. Commonly found in Gastropoda, Bivalvia and Scaphopoda, while Cephalopoda and many freshwater gastropods lost it through direct development with yolky eggs. Trochophore precedes veliger, connecting annelid-mollusc shared ancestry within Spiralia, indicating indirect marine development and dispersal before metamorphosis into benthic juvenile stage with shell.

Ref: NCERT Class 11 Biology, Chapter 4; Brusca & Brusca, Invertebrates, Chapter 16: Molluscan larvae

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Animal Classification

Gastropods show asymmetry due to

Gastropods are fundamentally bilaterally symmetrical larvae but become secondarily asymmetrical through embryonic torsion occurring during veliger stage, involving 180-degree anticlockwise rotation of visceropallium, mantle and shell relative to head-foot axis. Detorsion secondarily reduces it in opisthobranchs and pulmonates. Torsion relocates mantle cavity from posterior to anterior dorsal position above head, induces chiastoneury crossing pleurovisceral connectives into figure eight, causes loss of right ctenidium and kidney. Differential growth and asymmetric retractor muscle contraction drive this morphogenetic event generating adult asymmetry characteristic of snails and slugs.

Ref: Campbell Biology, 12th ed., Chapter 33: Gastropod torsion; NCERT Class 11 Animal Kingdom

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