IS A MOLLUSK AN EVOLVED BENT METATROCHOPHORE - A HISTOCHEMICAL INVESTIGATION OF NEUROGENESIS IN MYTILUS (MOLLUSCA, BIVALVIA)

Acetylcholinesterase (AChE) activity, a marker of neural differentiation, was histochemically localized in embryos and larvae of Mytilus galloprovincialis Lamarck and M. edulis L. (Mollusca: Bivalvia). The results show that: (1) the first AChE-active cells develop as two bilaterally symmetrical, postero-dorsal pioneer sensory neurons and their supporting superficial cells; (2) the pathways of their pioneering longitudinal axons are the same as those of two bilaterally symmetrical nerve cords which differentiate later in a posteroanterior direction; (3) in the conchostoma larva the posterior neurons and their associated superficial AChE-active cells give rise to two ciliary sensory-like organs which are the earliest rudiments of pedal ganglia and primary byssus glands; (4) three pairs of post-trochal gangliar rudiments, most probably visceral, parietal and pleural ganglia, develop on the same longitudinal nerve cords in the trochophore larva; (5) starting from the veliger stage, the presumptive foot rotates on the ventral side and migrates forwards, so that this gangliar chain is bent and the viscero-parietal ganglia become localized dorso-posteriorly to the pedal ganglia; 6) the cerebral ganglia differentiate together with the apical organ and the nerve network of the velum after the first pedal nerve rudiments have been detected. The findings modify the view of a tetraneural pattern of the molluscan nervous system and support a homology of the post-trochal gangliar chain of molluscs with the ventral nerve cord of annelids. A hypothesis on the phylogeny of annelids and molluscs, and the possible roles and evolution of the posterior sensory organs, are discussed in the light of the trochaea theory.