A description is given of the configuration of the surface membrane in the 3 zones of the polarised system of oocyte and follicle cells. Thus the cell contacts and relationships between these cells are considered and speculation on the significance of surface morphology in the passage of substances into the enlarging oocyte, is included.

The structure and some aspects of the development of the eye of Succinea putris were studied with the aid of the electron microscope. The eye is of the closed vesicle type and is composed of retina, cornea, vitreous body, lens and optic nerve. Three different types of cell are to be found in the retina: (1) the small elongated pigment cell with an avoid nucleus, many pigment granulae and short microvilli at the apical end of the cell; (2) the sensory cell type I with a large irregular nucleus, long microvilli, which extend to under the surface of the lens, a large number of light-cored vesicles, 700 A in diameter and the axon; (3) the elongated slender sensory cell type II with many dense cored vesicles, several pigment granulae in the distal region of the cell and short irregular microvilli at the apical end of the cell. This type is few in number. Two results of the study of the embryonic eye are described: the cornea cells differ from those in the adult eye in the nucleus-cytoplasm relation and the optic nerve is smaller than in the adult eye.

We have examined changes of postsynaptic potentials and of pattern activity of the identified silent and oscillatory snail neurones in Helix pomatia during conditioning. Local changes of EPSP or IPSP have been recorded during association following the first stimulus in the silent cells, whereas spike discharges could be observed in response to the 2nd stimulus. In the oscillatory neurones changes of pattern activity have been recorded following the 2nd stimulus, while the first stimulus proved to be ineffective. The formation of temporary connections of snail neurones seemed to be a specific phenomenon, because it was necessary to pair stimuli of different inputs for the development of these modifications. These plastic changes seemed to depend on the interstimulus as well as on the intertrial intervals. Our experimental data underline the probable role of the stimulus parameters and of the electrical properties of neurones during the formation of learned neuronal responses.

The regeneration of the cerata was studied in Aeolidiella soemmeringi (Leuckart, 1828) by light and electron microscopy. After amputation of the tips of the cerata the edges of the epidermis and of the hepatopancreas are drawn together by muscular contraction. The restitution of the cnidosac and of the hepatopancreas begins within groups of reserve cells, which are to be found in the hepatopancreas at each time. The restitution of the epidermis starts from undifferentiated replacement cells in the epidermis. Undifferentiated cells, which probably are fibroblasts and amoebocytes form a loose, spongy blastema. The amoebocytes become more and more loaded with phagocyte material and often show a pycnotic nucleus with very densely packed chromatin, while the fibroblasts predominantly form the fibrocytes of the mesenchym of the growing ceras. Thus it seems, that to a certain extent each tissue, i.e. ectoderm, entoderm and mesoderm, retains its individuality and is repaired from its own elements.

In Mytilus edulis 5-Ht induced relaxation of a muscle in catch is preceded by an increase in 3',5'-AMP content. In vitro two proteins of the contractile apparatus are phosphorylated by 3',5'-AMP dependent protein kinases. The 295000 dalton protein cannot be identified, the other one is paramyosin. Phosphorylated paramyosin inhibits actomyosin ATPase of smooth mollusc muscles at low and high Ca++ concentrations.

In the present paper, the effects of reserpine (RSP) and chlorpromazine (CPZ) on the neurosecretory cells of a saline treated freshwater bivalve, Indonaia caeruleus have been tracked. After saline (0.1 ml of 1.5% NaCl per animal) treatment A and B cells of cerebral and visceral ganglia showed significant increase in cell area, nuclear diameter and decrease in the NSM, while administration of RSP (0.50 mg per animal) and CPZ (0.25 mg per animal), following the saline injection completely inhibited these changes. Physiological implications of these changes are discussed in the light of neurosecretory dynamics.

The thirty Yellow Cells of Lymnaea show single, double and other extra spike modes of firing. Yellow Cell bursts consist of various combinations of single, doublet and triplet spikes whose number per burst varies spontaneously. Single spike firing modes of activity can be converted into doublets or bursts (and vice versa) by applying steady currents of the appropriate polarity. Spike activity is basically endogenous although it is modulated by low frequency synaptic input originating from within the brain. Interburst interval is affected by the number of spikes occurring in the preceding burst. This varies spontaneously or can be induced by applying appropriately timed current pulses or occurs following synaptic input. Excitatory synaptic input often induces bursts which far exceed the duration of the input and which are followed by long periods of inhibition.

The properties of 2 giant electrically coupled neurones (A10 and P1) identified in the visceral and right parietal ganglia of Lymnaea stagnalis were examined. The active and passive electrical parameters of the neurones, as well as the junction between them were measured. The main peripheral and interneuronal connections of the neurones were demonstrated using both electrophysiological and morphological methods. It is shown that the coupled cells are not neurones of the same function, but they are asymmetrical ones. This finding is supported by the following results: (1) the axonal pathways of neurones A10 and P1 are different; (2) there are significant differences in their afferent and efferent connections; (3) though the electrical junction between them is bidirectional, the junctional electrical characteristics prefer P1-A10 transmission. According to the electron microscopic results both neurones are possible neurosecretory cells. The differences demonstrated between the 2 giant neurones may have significance concerning their role in a special neuronal network.

Isolated ganglia possess the ability to concentrate tryptamine from an external medium by a process which is temperature sensitive and independent of sodium and other cations. Kinetic analysis of the accumulation process showed the influx of tryptamine to be a single mechanism with Km and Vmax values of 1.4 X 10(-4)M and 5 X 10(-8) mole/g/min. The influx of tryptamine is an unspecific process and is insensitive to a number of metabolic inhibitors and various analogues. The process of tryptamine influx is thus similar in principle to the low affinity uptake mechanism for 5-HT (see Osborne et al., 1975). The present data, which include some experiments on the release of 5-HT and tryptamine, are discussed from the point of view of a functional role for 5-HT and tryptamine in the snail CNS.

In Egeria radiata (L.) and Mutela dubia (Gmelin) decerebration does not result in tonus of the posterior adductor muscle and each species continues to exhibit its characteristic rapid and slow rhythms. Excitatory and inhibitory nerve pathways originating in the cerebral ganglia terminate on the visceral ganglia. The cerebral ganglia alone do not exhibit any rhythm; the anterior adductor muscle remains relaxed after excision of the visceral ganglia. The mid-dorsal and the anterior lobes of the visceral ganglia in Scrobicularia plana (Da Costa) control all adductor activity. Groups of potentials in the posterior adductor nerves originate from the different lobes and are separate physiological mechanisms.

In early September most of the neurons of the buccal ganglia of Helix pomatia contain neurosecretory material as membrane bound granules. There is only one, in exceptional cases 2 types of granules per cell. This suggests that different types of granules do not change into one another, and that each granule type contains a different secretory product. One granule type contains PAF-positive neurosecretory material, another one catecholamines, but most of the granules cannot be associated with special substances. The identified giant neurons B1-B4 contain granules in less density than the smaller neurons. B1 and B2 resemble each other in their granule type, whereas both B3 and B4 differ from B1 and B2.

Using convential mechanical and electromyographic recording methods 2 distinct types of neurogenically elicited activity can be observed in the penis retractor muslce (PRM) of Helix pomatia: (1) rhythmic, phasic contractions correlated with single or a few compound action potentials and (2) intervening, strong, prolonged contractions accompanied by sustained, high frequency electrical muscle activity. The 2 distinct types of muscle activity which seem to play a part in the normal behaviour of the PRM in the intact animal are mediated by both the central nervous system and peripheral neurons. While central neuronal structures are involved in causing the strong, prolonged contractions, the phasic activity is initiated by peripheral neuronal structures located at the proximal end of the PRM. There is evidence that the transmission of the excitation at the neuromuscular level of central and peripheral origin is mediated by ACh.

The mechanisms underlying the temperature response of the resting membrane potential (RMP) were investigated in 3 identified neurons of the buccal ganglion of Helix pomatia. Lowering the temperature evoked a decrease of the RMP and an increase in membrane resistance, and vice versa. The temperature response of the RMP had an equilibrium potential of ca, -60 mV. It is essentially evoked by changes in the potassium conductance. Indications of an electrogenic sodium transport were not detected.

The fine structural and cytochemical characteristics of sensory nerve cells have been studied in the lip of Helix pomatia. A ruthenium red positive cuticular layer was found on the surface of the sensory epithelium. Among the undifferentiated epithelial cells two types of sensory dendrites were observed, namely ciliated and non-ciliated ones. A large amount of smooth surfaced endoplasmic reticulum, microtubes and ribosomes were present in the neuroplasm of the sensory dendrites. However, rough surfaced endoplasmic reticulum, mitochondria, and electrodense bundles of long filaments were characteristic in the simple epithelial cells. The cell bodies of the sensory dendrites lie subepithelially among the muscle cells and they generally contain empty or dense core vesicles.

The axonal pathways and the synaptic inputs of the identified neurons B1 through B3 in the buccal ganglia of Helix pomatia were studied. The axons of neurons B1, B2 and B3 were found to run invariably within the ipsilateral posterior oesophageal nerve, ipsi- and contralateral salivary gland nerves, and ipsilateral cerebrobuccal connective, respectively. Synaptic responses could be elicited by stimulation of most of the nerves of the buccal ganglia. These consisted of an early depolarization which was most frequently followed by a longlasting de- or hyperpolarization. The shape of the synaptic response proved to be related to the different neurons.

Changes in the tentacle reflex pathway of the pond snail Lymnaea stagnalis induced by peripheral nerve injury were studied with behavioural and electrophysiological techniques. After nerve injury regeneration of sensory axons is obtained in 6-12 days, suggesting an axonal outgrowth at a rate of 1 mm per day. Recovery of the tentacle reflex takes much more time indicating that synaptic efficacy is affected considerably by the period of sensory deprivation following nerve injury.

Extraocular photosensitivity in Aplysia fasciata was studied in the skin and in the central nervous system (CNS). Local illumination causes contractions of the muscles of the body wall, which are obviously mediated by the peripheral nervous system (PNS). Afferent sensory activity is supposedly mainly dependent on stretch reception. Light-induced peripheral reflexes habituate after repetitive stimulation in preparations in which the CNS is present. In preparations without CNS light-induced contractions are remarkably stronger and do not habituate after repititive stimulation. Central responses to peripheral stimulation could be evoked by both "light on" and "light off" stimulation, indicating that 2 types of photosensitive elements are present in the periphery. Observations on isolated CNS-preparations revealed that in the central ganglia photoreceptive elements are also present. Here, too, elements responding to the onset as well as elements responding to the offset of light have been detected.

Cell body volume and Alcian blue-Alcian Yellow staining properties of neurosecretory neurones in the brain of Lymnaea stagnalis were compared for snails kept in de-ionised water and standard tapwater. In the same experiment, the ionic content of the blood, blood volume and body weight and environmental ionic composition were measured. Five days of immersion in de-ionised water resulted in significant decreases in body weight, blood volume and blood, Na+ and Cl- concentrations but no change in blood Ca2+, K+,and HCO3- concentrations, compared with controls. No consistently significant differences across the 5 day period were found in cell body volumes for Dark Green Cells, Yellow Cells or Light Green Cells (used as a control) when these volumes were compared for large numbers of cells from snails kept in de-ionised water and standard tapwater. However, the number of Yellow Cells which could be counted in snails kept in de-ionised water was lower than the number from standard tapwater by day 2 of the experiment and lower for Yellow-green Cells by day 5. We interpret this lower number to be the result of depletion of Alcian blue-Alcian Yellow stained neurosecretory material in these cells which made them impossible to distinguish. This was confirmed by examination of visceral Yellow Cells which could be identified on the basis of known location close to the visceral-right parietal connective.