Tusk Shells (Scaphopoda)

The Scaphopoda, or tusk shells, are a basal form of mollusk which filter feed on detritus and foraminifera from the surface of soft sediments.  The muscular foot (1P) anchors the anterior end of the body in the sediment while the apical (narrow) end protrudes the surface.  Water is drawn into the mantle cavity, which has no gills, via the apical opening using cillia.  Food particles are sorted from sediment before a muscular contraction ejects  the water back out the same opening.  This diagram shows an adult laterally within its shell (1) and ventrally without a shell (2) where blue represents nervous tissue, red is blood vessels, green is gonad and yellow is the digestive system which is shown isolated in higher magnification (3).  Further illustrations show a detailed progression of development (4-11).

Anodonta cygnea (freshwater mussel)

Commonly used as an examplar of it’s family, this fresh water mussel has a wide range across the whole of northern europe from the British Isles across to Siberia and down to Northern Africa.  There are many synonyms for this one species.  The top diagram shows the musculature of the mussel including the anterior and posterior adductor muscles used to keep the valves together (A.Ad & P.Ad), retractor muscles used to retract the foot (A.R & P.R) and the foot protractor muscle (Pr).  The middle diagram is of the scarring from muscles left behind in a shell once the soft tissue has been removed.  The bottom diagram focuses on the dorsal edge of the mussel to show the pericardium (Pc) a clear window which reveals the heart (Au & V) and the Keber’s organ, also known as the Organ of Bojanus (O.Bj).

Mytilus / Pecten

Generalised internal structures of two bivalve genera Mytilus (mussels) and Pecten (scallops) with left valve removed.  Structures are not labelled but easy to describe from the illustrations are the gut (yellow tubes with bulbous swellings), adductor muscle (large round pink structure with brown dots), gill (green on Mytilus grey on Pecten with lines running perpedicular to the length of the structure).

Gastropod Larvae

This chart compares development of a generic gastropod larva (fig. I & II) illustrating the velum and formation of the mantle cavity and operculum.  The third image is the larva of a sea butterfly, the classification of which remains confused to this day.  Sea butterflies were thought to be similar to sea angels because of their flapping appendages developed from the food however these two groups are now thought to be quite different.  Note the position of the forming mantle cavity relative to the operculum and velum between I & III, the sea butterfly has not undergone torsion.