• Intermediate Host Snail: In the Indian subcontinent, the freshwater pulmonate snail Gyraulus convexiusculus (family Planorbidae) serves as the obligate intermediate host transmitting both Gastrothylax crumenifer and Olveria indica to ruminants.
• Life Cycle Dynamics: Miracidia hatch from amphistome eggs and penetrate G. convexiusculus, undergoing sporocyst and redial generations before shedding amphistome cercariae (pigmented cercariae) that encyst on aquatic vegetation.
• Contrasting Snail Hosts:
• Indoplanorbis exustus: Major snail host for Paramphistomum cervi, Schistosoma nasale, and Schistosoma spindale.
• Lymnaea luteola / acuminata: Intermediate hosts for Fasciola gigantica and Orientobilharzia bomfordi.
• Melanoides lineatus: Prosobranch snail transmitting certain avian heterophyid and microphallid trematodes.

Correct option: (2) Gyraulus convexiusculus

• Class Gastropoda: Snails and slugs belong to the class Gastropoda (phylum Mollusca), the largest and most ecologically diverse class of molluscs.
• Distinguishing Characteristics:
• Torsion: 180° counter-clockwise rotation of the visceral mass during larval development, placing the mantle cavity and anus anteriorly.
• Asymmetry & Coiled Shell: Secondary bilateral asymmetry associated with a spirally coiled, univalve calcareous shell (or reduced/absent shell in slugs).
• Contrasting Molluscan Classes:
• Scaphopoda: Tusk shells (tusk-like tubular shell open at both ends).
• Amphineura (Polyplacophora): Chitons with 8 overlapping dorsal calcareous plates.
• Monoplacophora: Primitive single-shelled molluscs with serially repeated internal organs.

Correct option: (2) Gastropoda