Lecture Notes 8: Mollusca - Biology Major

Girdle: Mantle proliferation lateral to tegmentum, over articulomentum.. Highly variable among species, w/bristles, scales, granules, hairs (some innervated), CaCO 3 spines (secreted extracellularly as in Aplocophora)Mantle cavity: All around head & foot, w/ 4-40 prs. of ctenidia, water in from anterior and lateral, exitposteriorly.Dig.: Subradular organ tests substrate before feeding. Radula 17 teeth per row, often strengthened by addition ofhematite. Most extracellular digestion, dig. gland to esophagus; stomach. Intestine: fecal pellet fm. Mostherbivorous grazers, however often w/ large portion of sessile fauna in diet; Placiphorella as trappingcarnivore.Coelom &c: pericardium & paired kidneys w/ ducts & own prd. pore; single, central, fused gonad w/ paired,separate gonoducts and lateral gonopores opening in pallial groove and independent of pericardium. 1ventricle, paired auricles.Nerv.: amphineurous: i.e. lacks ganglionation & has pedal & pallio-visceral cords. Sense: aesthestes function?,eye.Repr.: Most dioceous, but Lepidochitona (~6 spp.), 2 spp. are hermaphroditic & selfing (very little male gonad asneed little). Some brood in mantle cavity.Dev.: All lecithotrophic, trochophore, add shell regions sequentially, 7 appears most clearly first but even theseappears to be sequential.MONOPLACOPHORAEarliest Cambrian to Mid-Devonian (350 Ma) until 1957. Proposed separate by Wenz 1938, Knight 1952 on basisof fossils based on multiple (5-8) muscle scars suggesting segmented bug. Neopilina galathea Lemche &Wingstrand 1957, collected in 1952, deep sea. Also Vema, shelf edge, canyons in CA. Now 30+ spp. Livingspp. lack muscle scars.Basal Conchifera or chiton sister? See Giribet et al. 2006.Metamerism: 5-6 pr. gills; 2 pr. auricles; 6 pr. nephridia; 2 pr. gonad; 8 pr. foot retractor muscles (this also inchitons & Babinka & may be primitive?); 10 prs. of nervous connectives between cords.Shell: limpet-like, flat, overhanging head. Fossils many w/ more elaborately coiled planispiral, etc. shells.