The evolution of the swimming habit in the Lamellibranchia

Abstract

The ability to swim is possessed by a wide variety of )Iollu.;,ca.ha ! he Dibranchiate Cephalopoda, where it is universal , the sh ell is reduced and int ernal and the necessary force is s upplied by the mu scles in the m antle the contraction of which drives out wat er through the siphon.The direction in which this i,-pointed controls the direc tion in which the animal m oves.The pe lagic squid s ar<: > most highly adapted for rapid m oYem ent, the body being stream-lined with lateral ; stabilizing fin s.Am ongst the Gastropoda, swimming is associated with the redu ction or modification of the shell , as in Carinaria and the Thecasomatou s Pteropoda , or it s complete absence, as in Ptero trachea and the Gymnosomatous Pteropoda.The m otive power is supplied by the foot which is modified to form lateral wings >> in the Pteropoda or a fin in the Heteropoda.In the LamelliLranchia , on the other hand , th e abili: ) io swim has been acquired by only a few species all of which , with the exception of Solenomya which may , as Drew (1906) has sh own , swim for short distan ces, are included in the Pectinacea and there confin ed to the P ectinidae, Amu ssiidae and Limidae.In these the shell is not reduced , although conditions in th e Galeommatidae reveal that this can occur in the Lamellibran chia tho ugh only in association with a commensal or para sitic life.. Swimming in the Lamellibranchia is carried out by movement s of the shell valves and has been mad e possible by comparatively minor modification s of the mantle, shell and adductor muscle.