HOMOLOGY OF THE PALLIAL AND PULMONARY CAVITY OF GASTROPODS

The development and morphology of the pallial and pulmonarycavities of various gastropods has been investigated using epoxy-resinserial sections. In the veliger larvae of Cellana sandwicensis(Patellogastro-poda), Gibbula adansonii (Vetigastropoda), Modulustectum (Caenogastropoda) and Ovatella myosotis (Pulmonata) theformation of the pallial cavity is nearly identical. After shellformation a shallow dorsal pallial groove develops beneath themantle edge. During the late veliger stage, the ectoderm formsa deep invagination along the bottom of the pallial groove onthe right side of the larva, giving rise to the pallial cavity.In the ellobiid O. myosotis the pallial cavity becomes the lung(=pulmonary cavity), without any major post-metamorphic modification.Thus, the lung of this species is clearly homologous with thepallial cavity of prosobranchs. The lung of pulmonates withveliger development, as well as of fresh water basommatophoransand stylommatophorans, can be shown to be homologous by comparisonof adult morphology. In contrast to previous views, the pulmonatelung should be regarded as truly homologous with the pallialcavity of prosobranchs and opisthobranchs. In the onchidiidpulmonate Onchidium cf. branchiferum, the larval pallial cavityshifts posteriorly after metamorphosis, where it gives riseto a lung and a cloaca. Contrary to previous interpretations,it can be shown that the onchidiid lung is homologous with atleast part of the pallial cavity. Smeagol climoi has only asmall pallial cavity and no separate lung. The previously described‘lung’ is shown to be a gland. The re-evaluationof the development and morphology of the pulmonate lung hasimportant systematic implications: (1) The pulmonary cavitydoes not represent a synapomorphic character of pulmonates.(2) The gymnomorphs cannot be separated from the remaining pulmonatesbased on lung development. (3) The lack of a lung in the smeagolidsmight give reason to reconsider this group's systematic placementwithin the pulmonates.