Molluscs are one of the most diverse groups of invertebrate animals - both in form and habitat. Therefore the study of molluscs has captured the interest of amateur and scientists alike for many centuries they have figured prominently in paleobiological and biological studies, and have served as study organisms in numerous evolutionary, biomechanical, ecological, physiological, and behavioural studies (Little, 1990)
Within the phylum Mollusca, snails and slugs make up the largest molluscan group, and have therefore been studied most extensively. They are so named because their foot spreads out when the animal crawls about (Little C 1990). The cerebral ganglion of several species (e.g., Euhadra peliomorpha, Aplysia californica, and Lymnaea stagnalis) secretes a neurohormone that stimulates the hermaphroditic gonad (the reproductive gland that contains both male and female characteristics); hermaphroditism is a common condition among molluscs. This gonadotropic peptide hormone is stored in a typical neurohemal organ until its release is stimulated.
For example, phototropic information detected by the so-called optic gland can direct the release of the gonadotropic hormone. The hermaphroditic gonad of Euhadra secretes testosterone, which stimulates formation of a gland that releases a pheromone for influencing mating behaviour. The optic gland of the octopus (of the class Cephalopoda) influences development of the reproductive organs on a seasonal basis. It is not known, however, whether any neurohormones are involved or whether this is purely a neurally controlled event.
All molluscs have a similar body plan with three main parts:
* a foot,
* a visceral mass,
* a mantle.
The foot is used for movement, the visceral mass contains most of the internal organs, and the mantle is a heavy fold of tissue over the visceral mass from which a shell may develop. The mantle cavity is an extension of the mantle, which contains the...