The radulae of Cronia margariticola (Broderip) and Morula musiva (Kiener) were examined on the specimens collected from southern Japanese waters. Although the radulae of these species have been already reported on by some authors, the present study reveals existence of sexual dimorphism and continuous changes with age in radula of both species. The length of radular ribbons and the breadth of rachidian teeth relative to shell length increase with growth. In the adult stage larger than 16mm in shell length, the rachidian and lateral teeth of males of these species were usually larger and grew more rapidly than those of females. Thus, the male radular characters may represent the secondary sexual characters. The actual number of radular rows is not sexually dimorphic and does not reveal differences between young and adult specimens. The rachidian teeth exhibit remarkable changes in shape as well as size with growth. As a rule, the rachidian teeth found in the young stage are of a complicated form with marked exterior (outer) denticles and marked marginal humps or cusps, and by gradual transformation, change to the simple form found in the adult stage. The rachidian teeth of fully grown males are extremely simplified due to such changes as well as sexual dimorphism. Investigation into previously published works combined with the known influence of sexual dimorphism, it can be shown that the rachidian teeth of Morula are characterized by atrophied exterior denticles and marginal cusps, which are intermediate in radular characters between Cronia and Thais.

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Mollusks living on leaves of Zostera marina were studied in Notsuke Bay of the eastern Hokkaido. One pelecypod and 15 gastropod species were collected by Marutoku net-B (45cm in mouth diameter, 0.33mm in mesh aperture) in the Zostera belt. Among them, the most abundant species was Barleeia angustata followed by Epheria decorata, Lirularia iridescens, Alvania concinna and Homalopoma sangarense in decreasing order. B. angustata, E. decorata and L. iridescens occurred throughout the year in most of the Zostera belt. Newly born juveniles of these three species came to appearance in June and grew rapidly during summer and autumn, so they approached their full-grown size in the next spring. Batillaria cumingii, Mitrella tenuis and Reticunassa fraterucula hypolia were abundantly found on the sandy or muddy bottoms of the bay, but they were very rare in the Zostera belt.

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Distributions and seasonal fluctuations of gastropod mollusks living on leaves of Zostera marina were studied in Notsuke Bay, Hokkaido, from May 1980 to May 1981. Barleeia angustata and Lirularia iridescens were more abundant in the central part of the bay than elsewhere. Epheria decorata was more abundant in the inner part of the bay. B. angustata and E. decorata were the two most abundant species collected and occurred throughout the year at all sampling stations. The total number of individuals of gastropods reached maximum in early summer when the density of Zostera marina was the highest, and fell to the minimum in winter. The seasonal fluctuations in numbers of gastropods synchronized well with that of Zostera marina.

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In Lake Tanganyika, 137 specimens of the mussel Moncetia lavigeriana were collected from August to early December 1983 and their shell length, sex and reproductive conditions were examined. Gravid females were observed during the present study. They have incubated 3000 to 22000 eggs in the inner gills. The larvae (glochidia) were found on the gills of fishes of 18 species. Among them, two predators on bivalves (Lobochilotes labiatus and Lamprologus tretocephalus) were the main hosts of glochidia. There may be a kind of symbiotic relationship between fish and mussel as fish feeds on mussels but rears their larvae.

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Under moist conditions, terrestrial pulmonates tend to increase their general behavioral activity. Hydration of these animals resulted in decreased hemolymph osmolarity and increased activity. In the dehydrated condition, this phenomenon is shown to be reversible. Using the giant African snail, Achatina fulica, we examined the induction of behavior electrophysiologically. From their morphological appearance and the characteristics of intracellular recordings, some giant neurons were identified on the surface of each ganglion. Using autoactive neurons, the effects of osmotic stress on electrical activity were examined by changing the bathing medium. As a result, higher electrical activity was obtained by hypertonic saline and lower activity was induced by hypotonic saline in isolated ganglia. On the contrary, in intact snails possessing the cerebral ganglion which contains the osmoreceptor, these effects became reversible. In a neuron of the parietal ganglion, electrical activity was induced by hypotonic saline and suppressed by hypertonic saline. These results correspond well with our theory on the induction of behavior in terrestrial pulmonates by hemolymph osmotic pressure.

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The effects of neuropeptides and neurotransmitters on the contraction of the cardiac and pharyngeal retractor muscles in two species of terrestrial snails were examined. Materials used were the giant African snail, Achatina fulica and the snail, Euhadra peliomphala. FMRF-amide (Phe-Met-Arg-Phe-NH_2) isolated from bivalve ganglia and proctolin (Arg-Tyr-Leu-Pro-Thr) were used as neuropeptides, and serotonin and octopamine were used as neurotransmitters. The heart rate was increased with the rise of temperature in both species. Besides, humidity was also shown to be the factor for the increase of the heart rate in E. peliomphala. The most fundamental characteristics of the cardiac muscle is its spontaneous rhythmical activity. FMRF-amide and octopamine induced rhythmical activity in hearts. These substances also produced the homogeneous tetanic contraction of the non-cardiac muscle, such as the pharyngeal retractor muscle. Although aspects of contraction were somewhat different, actions of FMRF-amide were greater than those of octopamine. Serotonin and proctolin also induced to contract not only the cardiac muscle but also the non-cardiac muscle. In these substances, the single contraction was induced in low concentration and it became tetanic with the increase of concentration. Together with the other physiologically active substances, FMRF-amide is presumably released into the hemolymph to provide long term maintenance and regulation of tone, rhythmicity and excitability of some muscles.

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