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Data on historical change of the Transbaikalian malacofauna in the Neopleistocene and Holocene is presented. Fossil shells from archeological excavations of the ancient settlements dating from the Neolithic period to Medieval and also from a drill hole of the Neopleistocene alluvial deposits were collected. In total nine species of bivalve molluscs from the families Margaritiferidae, Unionidae, Limnocardiidae, Glycymerididae, including one marine species, and two gastropod species from families Viviparidae and Planorbidae were identified. The time of the existence of each fossil species was determined by radiocarbon dating. It was found that the species ranged in age from more 50,000 and 2,080–1,210 years ago. Five species inhabited the Transbaikal region and are locally extirpated in the present. Their disjunctive ranges in the past included southern Europe and Western and Eastern Siberia to Transbaikalia and in the east to Far East and Primorye of Russia. The time of existence and extirpation of the thermophilic species of genera Adacna, Planorbis, Lanceolaria and Amuropaludina corresponds to cycles of the warming and cooling in Pleistocene and Holocene according to regional climate chronological scales. It was possible to separate these species as indicators of paleoclimate. Change of the species composition of the malacofauna of region connected with natural cycles of climatochrons in the Pleistocene and Holocene is the appearance of the climatogenic succession. In the course of this succession the disappearance of the stenothermal species occurred on a regional level and decreasing their global ranges.

Terminal portions of the male copulatory apparatus of Planorbis planorbis, Segmentina oelandica, and Anisus vortex were studied using whole-mount preparations, serial semi-thin sections, and transmission electron microscopy. In the latter species, stylet formation was investigated at several stages of postembryonic development. Organization of the penial distal portion in the species studied varies greatly. In P. planorbis, the distal end of the penis lacks developed papillae and is armed with a stylet built up of the covering epithelial cells of the penis proper. In A. vortex, the stylet is formed by the secretory activity of the middle cells of the distal portion of the penis. To the time of maturation, the cells encompassing the stylet are broken down exposing its solid chitinous structure and characteristic shape. In S. oelandica, the distal end of the penis bears the long probably flexible papilla with the characteristics of an internal ‘skeleton,’ organized as a line of connective tissue cells and a...

Four freshwater pulmonate species (Lymnaea ovata, L. stagnalis, Physa acuta, Planorbis leucostoma)were living in several watercress beds known for their relationships with human cases of fasciolosis, whereas L. truncatula was never found. The aims of these studies were to determine the prevalence of natural infections with Fasciola hepatica in snails and to verify if these species might ensure the full larval development of this trematode (with cercarial shedding] when they were experimentally subjected to F. hepatica only, or to co-infections with an other trematode species. Investigations were so carried out in six snail populations living in watercress beds (including three for P. acuta) and in four others originating from three brooks or a pond (as controls). Snails naturally infected with F. hepatica were found in two watercress beds inhabited by L. ovata (prevalence of infection: 1.4 %) and P. leucostoma (0.1 %), respectively. The L. ovata from the watercress bed could be infected at a higher size than those from the control population and the prevalence of this infection was greater in the bed population. Similar findings were noted for L. stagnalis. Despite single or dual infections, the results obtained with the four populations of P. acuta were unsuccessful. In contrast, the co-infections of young P. leucostoma with Paramphistomum daubneyi and F. hepatica resulted in the shedding of some F. hepatica cercariae. According to the authors, the occurrence of fasciolosis in these watercress beds would be the consequence of frequent natural encounters between parasite and snails (L. ovata, L. stagnalis), or of co-infections with P. daubneyi and F. hepatica (P. leucostoma). In watercress beds only colonized by P. acuta, a lymnaeid species would have ensured the larval development of F. hepatica but it would have been eliminated by P. acuta, as this last species was known to be invasive and could colonize open drainage ditches on siliceous soil.

A description of Biomphalaria obstructa (Morelet, 1849), based on specimens collected at its type locality - isla del carmen, state of Campeche, Mexico - is presented. The Shell is small, 13 mm in diameter, 3.5 mm in width and with 5.75 whorls in the largest specimen, thin, moderately lustrous and translucent, horn-colored. Whorls increasing regularly (neither slowly nor rapidly) in diameter, rounded on the periphery side, bluntly angular on the left. Suture well-marked, deeper on the left. Right side widely concave, with first whorl deeply situated and partly hidden by the next. Left side shallower than right one, largely flattened, with first whorl plaintly visible. Aperture roundly heart-shaped, usually in the same plane as the body whorl but somewhat deflected to the left (less frequently to the right) in some specimens. Peristome sharp, seldom blunt; a distinct callus on the parietal wall. A number of young shells develop one set (seldom more) of apertural lamellae which tend to be resorbed as the shell grows. Absence of renal ridge. Ovotestis with about 70 mostly unbrached diverticula. Seminal vesicle beset with well-developed knoblike to fingerlike diverticula. Vaginal pouch more or less developed. Spermatheca club-shaped when empty, egg-shaped when full, and with intermediate forms between those extremes. Spermathecal body usually somewhat longer than the duct. Prostate with 7 to 20 (mean 12.06 ± 2.51) usually short diverticula which give off plumpish branches spreading out in a fan shape and overlapping to some extent their immediate neighbors. Foremost prostatic diverticulum nearly always partially or completely inserted between the spermathecal body and the uterine wall. Penial sheath consistently narrower and shorter than the prepuce. Muscular coat of the penis consisting of an inner longitudinal and an outer circular layers. Ratios between organ lengths: caudal to cephalic parts of female duct = 0.55 to 1.37 (mean 0.85 +- 0.17); cephalic parte of female duct to penial complex = 1.36 to 2.81 ((mean 1.90 +- 0.33); penial sheath to prepuce = 042 to 0.96 (mean 0.67 +- 0.13). Comparison with Morelet’s type specimens of Planorbis orbiculus and P. retusus points to the identity of those nominal species with B. obstructa.

Cases of fasciolosis in ruminants have been recorded in several French farms in the absence of Lymnaea truncatula, which is considered the only snail intermediate host in western Europe. These farms harbored other species of freshwater snails in large numbers (Lymnaea glabra, Physa acuta, or Planorbis leucostoma) and, in many cases, had cattle or sheep infected by another trematode (Paramphistomum daubneyi). These other freshwater snails may serve as intermediate hosts for F. hepatica due to a coexisting infection with P. daubneyi. We have demonstrated that L. glabra, either infected with F. hepatica alone or coinfected by P. daubneyi, was capable of developing a F. hepatica infection. A broader range of L. glabra size classes (up to 10 mm in height) were susceptible to infection if simultaneously infected with P. daubneyi. Planorbis leucostoma can only serve as an intermediate host for F. hepatica, if infected with P. daubneyi. Lastly, P. acuta smaller than 4 mm cannot serve as an intermediate host. These results may explain, in part, the maintenance of low-level F. hepatica infections in the absence of the normal intermediate host, L. truncatula.

1. In the intact pond snail Planorbis corneus, a rapid tilt in any plane evoked a defense reaction consisting of a fast movement of the shell towards the head, shortening of the foot, inhibition of locomotion and of rhythmical feeding movements. This reaction was similar to the first phase of the general defense reaction of Planorbis to cutaneous stimulation. 2. A method has been developed for inclination of the isolated CNS in space (up to 90 degrees) and simultaneous intracellular recordings from different neurons. 3. The statocyst receptor cells (SRCs) responded both phasically and tonically to the tilt. The SRCs differ in their spatial zones of sensitivity. 4. Essential manifestations of the defense reaction to the input from statocysts could be observed in the in vitro preparation of the CNS isolated with statocysts. Both tilting of the CNS and electrical stimulation of individual SRCs elicited an excitatory response in numerous neurons from different ganglia, including motor neurons (MNs) of the columellar muscle. This response was of "all-or-none" nature, and could be evoked by electrical stimulation of any SRC. The response was followed by a long (10-20 s) period of refractoriness. 5. Activation of SRCs resulted also in excitation of the giant dopaminergic cell in the left pedal ganglion (related to the control of respiration), in inhibition of the feeding rhythm generator, and in inhibition of the pedal neurons responsible for activation of the ciliary locomotor system. 6. Combined stimulation of two inputs able to evoke a defense reaction, i.e., those from the statocyst and from cutaneous nerve, revealed a strong interdependence of their central effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolated buccal ganglia of Planorbis corneus are capable of generating a feeding rhythm. In the present work, "rhythmic" neurons of different groups (see Arshavsky et al. 1988a) have been extracted, by means of an intracellular microelectrode, from the buccal ganglia. (1) After extraction, efferent neurons of groups 3, 5, 7, 9 and most group 4 neurons generated repeated spikes at a frequency controlled by a polarizing current. Any periodic oscillations, similar to those during feeding rhythm generation, were absent in these isolated neurons. It is concluded, therefore, that these neurons are "followers", that is, their rhythmic activity before extraction is determined by synaptic inputs from other neurons of the ganglia. (2) Isolated interneurons of groups 1 and 2 generated slow periodic oscillations similar to those observed in these neurons before their extraction. Subgroup 1e neurons generated smoothly growing depolarization accompanied by increasing spike activity; this depolarization was periodically interrupted by abrupt hyperpolarization, after which a new cycle started. Subgroup 1d neurons periodically generated short series of spikes. Group 2 neurons periodically generated a rectangular wave of depolarization with spike-like oscillations on its top. These results suggest that feeding rhythm generation in Planorbis is based on the endogenous rhythmic activity of group 1 and 2 neurons. (3) A pulse of hyperpolarizing current injected into an isolated neuron of subgroup 1e stopped the growth of depolarization in the neuron and reinitiated the process. This property as well as the character of the synaptic interactions of the interneurons (group 1 neurons excite those of group 2, while those of group 2 inhibit group 1 neurons; Arshavsky et al. 1988b) determine the alternating activity of groups 1 and 2.

Double-barrelled, ion-selective microelectrodes (ISMEs) have been used to measure the intracellular free concentrations of K+ ([K+]1), Na+ ([Na+]1) and Cl- ([Cl-]1), together with membrane potentials (EM), from single salivary gland acinar cells of the pond snail Planorbis corneus. After adjustments had been made for the cross-sensitivities of the ion-exchangers to other intracellular ions, the mean concentrations were estimated to be: [K+]1, 42·9mmol1-1; [Na+]1, 2·4mmol1-1; and [Cl-]1, 10·3 mmol 1-1. The mean Nernstian equilibrium potentials for K+, Na+ and Cl- were calculated to be —88 mV, +74·4mV and —41 mV, respectively. The basolateral membrane of Planorbis salivary cells appears to be permeable to K+ and Na+ under resting conditions, because blocking the electrogenic Na+/K+ pump with K+-free saline or ouabain revealed the presence of a large passive efflux of K+ and an influx of Na+. Salivary gland cells also lose intracellular Cl- rapidly in CD-free saline (extracellular Cl- replaced by sulphate) which, along with other evidence, indicates a substantial resting permeability of the salivary cell membrane to Cl-. Stimulating gland cells with 10−4 mol 1-1 acetylcholine (ACh) led to a depolarization of EM1 a rise in [Na+]1 and a fall in [K+]1. This was followed by a transient hyperpolarization of EM and a recovery of [Na+]1 and [K+]1 to their original levels. There was no evidence that [Cl-]1 changes after stimulation with ACh. The mechanism of action of ACh on Planorbis salivary gland cells and its relevance for secretion are discussed.

In the buccal ganglia of the freshwater mollusk Planorbis co~'neus protein synthesis takes place most intensively in thebodies of nerve cells. Protein synthesis in giant neurons takes place with equal intensity both in cytoplasm and nucleus, 60% of the total mass of sulfur-containin g protein being produced by the cytoplasm. Protein leaves the neuron body principally along the axon. It migrates along the axon at a rate of about 0.2mm/day. Giant neurons of the buceal ganglia are characterized by very high electrical activity, giving way periodically (at intervals of 3-5 sec) to deep and prolonged (about 1 see) inhibition. This type of periodic unit activity continues substantially unchanged for up to 20 hr if the surrounding isotonic solution is constantly renewed.

1. (1) Two schistosomes were discovered in wild rodents in the south-east of the Belgian Congo (Province of Katanga): S. rodhaini Brumpt, 1931 ; and S. mansoni var. rodentorum Schwetz, 1953a , Schwetz, 1953b . Both belong to the group of S. mansoni, by virtue of their anatomy as well as by the identity of their intermediate hosts (Planorbis). 2. (2) S. rodhaini differs from S. mansoni in the structure of its eggs and in its definitive host. Instead of a lateral spine, the eggs of S. rodhaini have a subterminal spine directed sideways, and a narrow appendix-like opposite extremity, turned in the opposite direction. S. rodhaini has as yet been found only in divers wild rodents, never in human beings. S. rodhaini foci were discovered in Elisabethville, Albertville and Sakania, and it is most likely that it exists elsewhere too. In Elisabethville and Sakania, the transmitters of S. rodhaini were the river Planorbis (P. pfeifferi), while in Albertville the transmission of S. rodhaini was effected by P. tanganikanus.Experimentally, other African (P. boissyi = P. alexandrina) and American (P. glabratus) planorbids were found to be able to transmit this schistosome. In Sakania, the only river present in this locality, was found to contain a mixed focus of S. rodhaini and S. mansoni. Mice infected with the local cercariae were found to pass eggs of S. rodhaini 30 days after their exposure to infection, and after the 50th day eggs of S. mansoni also. In Albertville, on the other hand, the foci of these two schistosomes are as a rule well distinct and separate. 3. (3) While searching for S. rodhaini in the wild rodents, we also found in these animals some very rare eggs with a lateral spine. We found these eggs associated with those of S. rodhaini in Sakania; and alone, in Albertville. Since this was an entirely new discovery, we devoted several months of research to it at the end of 1952, chiefly in Albertville, and, briefly, have arrived at the following conclusions We are dealing with an infection that is both rare and distinct. Out of 480 rodents coming from 15 rivers (rivers containing P. pfeifferi), we found only 18 infected specimens— i.e. an incidence of 4 per cent. As we have already pointed out, this is a very light and inconspicuous infection, usually signified by the finding of several broken or empty eggs in the liver. Only in very few cases did we discover living eggs, and as for the adult worms, they were also a rarity. Although these eggs, too, had a lateral spine, they appeared to us from the very first time we saw them, rather different from the normal eggs of S. mansoni—both in the form of the egg (elongated, with a narrow extremity), and in the form of the spine (a solid spine with a wide insertion, separated from the rest of the egg by a notch). 4. (4) In view of this particular structure and especially of the hitherto unknown definitive host, we have decided to call this schistosome—S. mansoni var. rodentorum. This new schistosome is of great practical interest, especially from the epidemiological and prophylactic viewpoint. Being adapted to definitive hosts which dwell in the same rivers that are frequented by the natives, the question arises whether this is not in reality the same parasite as S. mansoni, modified in its morphology and biology by its adaptation to different hosts—to the wild rodents first, and to human beings thereafter.

1. 1. 5-HT and 5-HIAA were detected in the circumoesophageal ganglia of Planorbis corneus . 2. 2. Pretreatment of the snail with d,l -5-HTP increased the amount of 5-HIAA present in the brain extracts. 3. 3. The metabolism of 5-HT by monoamine oxidase would appear to be a method of inactivating 5-HT in the snail brain.

The best figures of the nervous system of these animals are those of Lacaze-Duthiers, in his Paper Sur les Otocystes des Mollusques, and in that Sur le Système Nerveux des Gastéropodes. By a careful study of the nervous system of these two gasteropods, chiefly by cutting sections, I have discovered one or two small points in their anatomy which, as far as I can make out, have hitherto escaped attention.These two types, though so closely allied, differ to a considerable extent in the arrangement and structure of certain of their ganglia; compare the figures given by Lacaze-Duthiers of the nervous system of Lymnæus on Plate xvii. in Volume i. of his Archives with that cf Planorbis on Plate iii. of the same volume. It will be seen at once that the cerebral ganglion of Lymnæus is far more complicated than that of Planorbis, consisting of five separate lobes, while that of Planorbis appears to be made up of not more than two.

Mice, fish, gastropods and polychaetes were injected with the venoms of 23 species of Conidae. The venoms of Conus magus, C. striatus and C. stercusmuscarium quickly immobilized fish and were lethal to mice. Lethal and near lethal doses of posterior duct venom from C. magus produced a spastic paralysis in mice whilst lethal and near lethal doses of anterior duct venom of C. magus, posterior duct venom of some specimens of C. striatus and the venom of C. stercusmuscarium caused flaccid paralysis of the skeletal musculature of mice. Venom from some specimens of C. striatus induced circling movements in mice. The venoms of C. omaria, C. aulicus, C. episcopis, C. ammiralis and C. tigrinus quickly paralysed the gastropod Melarapha scabra. The polychaete Phyllodoce malmgremi was paralysed or partially paralysed by the venoms of C. litteratus, C. millepunctata, C. pulicarius, C. figulinus, C. arenatus, C. imperialis, C. virgo, C. rattus, C. eburneus and C. planorbis. Analysis of well-authenticated records of human injury inflicted by Conidae supports the contention that only piscivorous Conidae pose a threat to man. The structure of the radular teeth of Conidae provides a means of distinguishing between piscivorous, vermivorous and molluscivorous species.

1. 1. Limiting external pO2's have been determined for full and zero saturation of arterial and venous blood of Planorbis corneus and the pO2 range for pigment function has been deduced. 2. 2. For oxygen uptake from the lung the pigment is functional over the pulmonary pO2 range 20–60 mm; this is precisely the region over which Planorbis pulmonary oxygen utilization exceeds that in Lymnaea stagnalis lacking a respiratory pigment. 3. 3. Cutaneous uptake is facilitated by haemoglobin down to a dissolved pO2 of about 30 mm. 4. 4. The oxygen capacity of Planorbis blood was found to range from 0.94 to 2.49 vol per cent; the question of a possible storage function is discussed.

In two widely separated places of Lake Bunyonyi at Chabahinga, near Kabale, and in Muko, we found the same three species of fresh water snails: 1. (1) Pl. pfeifferi Krauss. 2. (2) B. coulboisi Bourg. and B. mutandaensis Preston. 3. (3) L. natalensis (undussumae) von Martens. Bulinus were the most abundant, Planorbis also numerous, and Lymnaea comparatively rare. Amongst the Planorbis (and Bulinus) of Chabahinga we found none infected with Schistosoma cercariae, but amongst the Planorbis at Muko we found two specimens shedding Schistosoma cercariae. Both Planorbis died before we had time to try an animal experiment. We did not examine the natives of Muko, but amongst those of Chabahinga, near Kabale, we found a few boys infected with S. mansoni. Infection, however, was very light, and none of the affected complained of any symptoms. S. mansoni thus exists at an altitude of 6,300 feet.

Summary. From the data presented in the previous pages it is evident that the large planorboid snails are divisible into three genera. Each has a form of genitalia peculiar to itself and different from any of the others. The radula does not afford characteristics of generic value, but may be used for subgeneric distinctions. The shells do not affprd tangible generic characters, but may also be used for subgeneric distinction. The groups as now understood may be distinguished as follows:— 1 Vergic sac appearing as an enlargement of the vas deferens, very small and narrow, about one-sixth as long as the preputial sac; verge very amall, not evertible, but discharging the spermatozoa into a channel on an appendage connecting with the spermatophore gland, the appendage performing the fnnction of a verge; no spermatophore duct; anterior and posterior retractor muscles attached to preputial sac…Planorbis. 2 Vergic sac very large, wide, one-third of to as long as preputial sac; verge evertible, large, discharging spermatozoa directly into feniale opening; spermatophoro gland forming a cup-shaped body with many foldn in the cavity; a spermatophore duct of varying length connecting spermatophore gland with vcrgic sac, into which it opons just above a muscular diaphragm separating the two sacs; one retractor muscle, which may be split into two or morc branches, attached to the preputial sac…Helelisoma. 3 Vergic sac very long and narrow, as long as the sac; verge long and slender, evertihle; no spermatophore duct; spermatophore gland represented by heavy pads on the walls of the preputial sac…Indoplanorbis. The genus Helisoma is divisible into four subgenera, three of which may be diagnosed anatomically:— 4 Vergic sac nearly as wide as preputial sac, with but a slight constriction between the two saes; duet very short, about as long as preputial sac; lateral teeth with mesocones sharply pointed…Helisoma. 5 Vergic sac as long as preputial sac, pyriform, much narrowed at the constriction between the two saes; duct very long, one and a half to more than twiee as long as preputisl sac; spormatophore gland with willls much thicker on one side; gland wider than high; mesocone of lateral teeth very wide and blunt; aperture of shell campanulato…Planorbella. 6 Vergic sac as in 2; spermntophorc gland with walks thickened on all sides; shell with apcrture not campsnulate…Pierosoma. 7 Genitalia unknown; shell vertically compressed, whorls smaller and more numerous, apertnre very oblique…Planorbina. A systematic arrangement of the genera and subgenera will appear as follows:— Genus Planorbis Miiller, 1774. Type, Helix cornea Linn. Mainly European in distribution. Genus Helisoma Swainson, 1840. Type, Planorbis bicarinutus Sowb. North and South America and West Indies in distribution. Subgenus Helisom Swainson, typical. Type, Planorbis bicarinutus Sowb. North America in distribution. Subgenus Planmhella Hald., 1842. Type, Planorhis campanulutus Say. United States and Canada in distribution. Subgenus Pierosoma Dall, 1905. Type, Planorbis trivolvis Say. North and Central America in distribution. Subgenus Planorbina Hald., 1842. Type, Planorbis olivaceus Spix. South America, Central America, Mexico, southern United States, and West Indies in distribution. Genus Indoplanmbis Annandale & Prashad, 1920. Type, Planorbis exustus Desh. Indo-China in distribution.

In the visceral ganglion of Planorbis the postsynaptic neurones of the characterized dopamine neurone are connected by non-rectifying electrotonic junctions. The coupling, which is reduced by stimulation of the dopamine neurone and by applied dopamine, may be important in the generation of burst activity. Specialized areas of close apposition of membranes in the neuropile are considered to be the morphological correlate of electrotonic coupling.

Publisher Summary This chapter discusses the peptide effects on cholinergic-dopaminergic transmission between identified invertebrate neurons. Molluskan nervous systems provide the opportunity to study the interplay of neurotransmitters in a circuitry of identified neurons. In studying the peptide effects in the water snail Planorbis corneus, especially on the giant dopamine neuron (GDN) of the left pedal ganglion, the focus was on two types of peptides, i.e., representatives of the α-MSH/ACTH family and vasopressin. Vasopressin had been reported to influence invertebrate neurons but its presence in Planorbis remains uncertain. In contrast, evidence has been obtained for the existence of neurons containing a compound related to α-melanotropin (α-MSH). The immunohistochemical evidence for the presence of an α-MSH-like compound and the observation of consistent and reversible effects of α-MSH on neurotransmission are suggestive of a mechanism that uses α-MSH or a related substance. If such a compound occurs in Planorbis, opioid peptides might also be present.

Fasciolopsis buski is an important human parasite which has been reported from China, India, Assam, Bornea, Sumatra, Cochin China and Tonkin. It is most frequently found in China where it has been reported from Canton, Hongkong, Swatow, Shanghai and other places. According to Goddard (1920) there is a very important endemic center of this parasite in the region of Shaohing in the Chekiang province. In Formosa it is not infrequently found in pigs, but no human cases have as yet been reported. I obtained specimens of Fasciolopsis buski from native pigs in Formosa which measured 25 to 40 mm. in length and 10 to 17 mm. in width. Eggs from these individuals were incubated and free swimming miracidia obtained. Penetration of these miracidia was obtained into Planorbis coenosus and the development of the cercaria was studied. Later encysted cercariae were fed to experimental animals and the stages in development to the adult condition were traced. These studies were carried on over a period of five years, and an outline of them has already been published (Nakagawa, 1921). In the present paper I will give a more detailed account of the stages in development. The development of the eggs of Fasciolopsis buski resembles that described by Leuckart for the eggs of Fasciola hepatica. The eggs (Fig. 1) fresh from the uteri of the worms are a light yellowish brown in color and measure 0.12 to 0.14 mm. in length (average 0.13 mm.) and 0.07 to 0.09 mm. in width (average 0.08 mm.). A small operculum is present. When these eggs are placed in a small glass dish and the water changed each day, the fertilized egg cell undergoes cleavage and the miracidium soon begins to take form. At first the development is rather slow but later becomes more rapid. During the development of the miracidium the surrounding yolk cells begin to dissolve and gradually disintegrate into what appear to be drops of oil. The end of the developing embryo toward the operculum is broader and develops into the anterior end of the miracidium, while the opposite end early becomes somewhat pointed. As the length of the embryo increases, the surface of its body becomes ciliated and a small protuberance appears at its anterior tip. Just back of this there develops an X-shaped pigmented eyespot. About this time slight extensions and contractions of the body of the embryo can be noted. These movements gradually increase until the miracidium finally pushes

The structure of the penis of members of the family Planorbidae has be:n considered by several workers. In this family, as well as in other families of the Basommatophora and the Stylommatophora, it has been used as one of the important genital structures in taxonomic studies largely because it reveals many generic and specific relations. In the classical work on the morphology of planorbids by Buchner (1891), the penis of members of the genera Planorbarius, Segmentina, Planorbis and Anisus, Bathyomphalus and Gyrautus were compared. Rao (1923) and also Baker (1933 and 1945) described the penis of Indoplanorbis in considerable detail. Soos (1935) dealt with the genitalia of Hungarian planorbids, particularly Anisus (Tropidiscus), Gyvaulus and Armiger and he, too, emphasized the i:rnportance of the penis in taxonomy. The presence of a "penial gland" in the North American genus Helisoma was first reported by Baker (1926) and he also found it later in several other genera of American, European, and Asiatic planorbids. This same investigator (1931) thought that the gland was in some way involved in the formation of the spermatophore and, consequently, called it the spermatophore gland. Later, Baker realized that this assertion was not in agreement with the structure of this organ. In 1945 he stated that the function of the "penial gland" "is insufficiently known but it seemed to him that the secretion of the gland might serve to dilate an opening through which the verge protrudes or that the gland might serve as an excitatory organ or sarcobelum. In this present study, special attention is given to the morphological and histological structure of the organ, as well as to the mating habits of the snails, in order to clarify how this gland functions. Helisoma trivolvis was used mainly for this work, but a comparison was also made with certain other members of this genus. Further studies on other genera will be published in a forthcoming paper.

Infection with the blood fluke Schistosoma mansoni is a highly prevalent disease in Africa, in the northern parts of South America and in the West Indies as far north as Puerto Rico. In the continents of Europe and Asia there is only one small focus, about the city of Aden, Arabia. The disease has apparently never been acquired on the North American continent (fig. 1). The sharp limits of geographic distribution of the disease are due in part to the complicated life cycle of this flatworm, which requires one of a number of species of snail (of the genera Planorbis and Australorbis) as intermediate host. After development within the snail, the schistosome emerges as a larva, the cercaria, which swims freely near the surface of fresh water streams and ponds and there is capable of penetrating human skin. Within the human host the growing worms make an extended migration through

trematodes was made in the study of schistosomiasis in Egypt by Manson-Bahr and Fairley (1920) who kept a record of the cercariae found in the snails which they dissected over a period of one year. They found the cercariae of the human blood flukes, Schistosoma haematobium and S. mansoni, present in the snails at all seasons of the year. The highest percentage of infested snails occurred during the month of December. They note also that the infestations found in the autumn months were all mature, whereas in the spring months immature cercariae were the rule rather than the exception. Their observations on the other larval trematodes found in the snails indicate the same general trend, for the snails were infested throughout the year, with the highest percentage occurring in December. Their study was made in a part of the world in which drought has a great influence on the snail population and where there is not a marked fluctuation in temperature from season to season.