Histological,histochemical, and ultrastructural investigation of the male copulatory apparatus of Haminoea navicula (Gastropoda,Cephalaspidea)

Due to its biological and systematic importance, the morphology and function of the male copulatory apparatus of Haminoea navicula, a Cephalaspidea gastropod mollusk, was investigated by light and electron microscopy. These systems are poorly understood in haminoids, but are often used in the taxonomy of the genus. In H. navicula, the male copulatory apparatus comprises the penis within a penial sheath, a seminal duct and the prostate with two lobes. The penis is a muscular structure with a tip covered by spikes formed by muscular cells. The penial sheath consists of muscular tissue folds lined by an epithelium. Below this epithelium, polysaccharide‐secreting cells and pigment cells were observed. A large number of vacuolar cells were found below the ciliated epithelium of the seminal duct. The proximal lobe of the prostate was formed by tubules that could be divided in basal, middle and apical zones, containing cells that secrete polysaccharides and proteins. The tubules of the prostate distal lobe contained a single type of secretory cells with vesicles that were stained by histochemical techniques for detection of polysaccharides and proteins. Ciliated cells were present along the tubules in both lobes of the prostate. This study revealed a complex prostate with five types of secretory cells, which secrete substances that should be involved in the maintenance and eventually also in the maturation of spermatozoa. A comparison with previous publications, shows that the male copulatory apparatus can differ substantially among cephalaspideans, even between H. navicula and non‐European species attributed to this genus.     

Origins and relationships of primitive members of the Opisthobranchia (Mollusca: Gastropoda)

An examination of possible prosobranch precursors of opisthobranchs and pulmonates suggests that archaeogastropods are poor candidates, as they lack the complex female glands which characterize all higher gastropods. Similarities between members of the Rissoacea and Cerithiacea with the Opisthobranchia and Pulmonata are here considered to be a result of parallel evolution. Much of the basis for suggesting phylogenetic aRinities between members of these taxa hinges on the supposition that pyramidellids are opisthobranchs. This contention is questioned, as the euthyneurous nervous system and hermaphroditic reproductive system share weak structural and positional homologies with opisthobranchs. The range of morphological variation within the Pyramidellidae should be more thoroughly investigated, as this taxon is poorly known. The reproductive system of members of the Littorinacea is largely homologous with that found in the least derived opisthobranchs and pulmonates. This, when considered in conjunction with other morphological and palaeontological evidence, suggests that the least derived mesogastropods, the Littorinacea, provide a more precise model of the opisthobranch-pulmonate ancestor among living prosobranchs. The specialization of the digestive system in littorinaceans suggests that morphological divergence has taken place following the cladogenesis of these taxa, and that a direct ancestor/descendent relationship cannot be implied from extant littorinaceans. The status of Acteon and other acteonids as archetypal opisthobranchs is questioned. Moditication 01 all morphological systems, with the exception of the shell and mantle complex, diminish the position of the Acteonidae as ancestral opisthobranchs. Members of the Ringiculidae more closely approach the ancestral form, but have undergone modification, as well. No extant opisthobranch retains all plesiomorphic character states. For this reason, in addition to the fact that contusion exists as to the plesiomorphic conditions of some characters within the Opisthobranchia, a hypothetical ancestral opisthobranch is characterized. It appears that parallel evolution, which confounds the relationships of certain prosobranchs and opisthobranchs, is also evident within the Opisthobranchia. Much of the confusion that has surrounded the determination of phylogenetic relationships within the Opisthobranchia relates directly to the high incidence of parallelism throughout the subclass.