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Hermaphroditism in Mollusca

1894; The Company of Biologists; Volume: S2-37; Issue: 145 Linguagem: Inglês

10.1242/jcs.s2-37.145.19

1477-9137

Paul Pelsencer,

Isotope Analysis in Ecology

Hermaphroditism is found to occur in every class of Mollusca except the Cephalopoda and Scaphopoda.1. Among Amphineura, in all the Neomeniidæ.2. Among Gastropoda, in four genera of Streptoneura (Valvata, Onchidiopsis, Marsenina, and Entoconcha), and in all the Euthyneura (Opisthobranchia and Pulmonata).3. Among Lamellibranchia, in various species of Pecten,Ostrea, and Cardium; in Entovalva, the Cycladidæ, the Poromyidæ, and all the Anatinacea.(1) Neomeniidæ.—In each of the two contiguous hermaphrodite glands of these animals the ova arise in the axial half of the organ, and the spermatozoa in the lateral half (1).(2) Cryptochiton.—According to Middendorf (2) this genus is hermaphrodite. I have, however, been able to obtain a specimen in alcohol of this form (C. Stelleri, from the North Pacific), and have found it to present a distinctly unisexual condition, as in Chiton. I have also satisfied myself that the sexes are separate in Chitonellus larvæformis (= fasciatus) and Cryptoconchus porosus (or monticularis). Thus the diœcious condition is universal in the Poly-placophora.(1) Valvata.—The hermaphrodite gland of this genus is formed of acini, all producing ova and spermatozoa (3).(2) Marsenina and Onchidiopsis (of the family Lamel-lariidæ).—While the hermaphroditism of Valvata has been recognised by a fair number of zoologists, that of the two genera just named has only been affirmed by Bergh (4).As the number of monœcious Streptoneura is extremely limited, an examination of these two genera was desirable in order that Bergh’s discovery might be confirmed and his statement of facts extended. I have only succeeded in obtaining a single specimen of Onchidiopsis greenlandica, and upon it I have made the following observations.The genital gland occupies the posterior part of the visceral mass. Its duct is provided with an accessory mass formed of closely packed, narrow, tubular cæca; it then bifurcates, and the more lateral branch (the right), after first receiving a large bent pouch (fig. 1, iv), immediately thickens before opening to the exterior, and again receives on its right side a large pouch of flattened form. The aperture of this branch is situated in the mantle cavity, in front of the anus and more to the right; it is the female orifice (vπ).The other branch immediately presents a glandular mam-millated mass with closely packed lobules, and then extends forward under the integument over the neck of the animal as far as a considerable projection of the body-wall on the right side, the penis, which it traverses from end to end (fig. 1, v).The structure of the reproductive apparatus thus differs from that of Valvata in the presence of glands upon the hermaphrodite portion of the duct, and by the absence of a differentiated “uterus “in the female portion.The glands of the hermaphrodite duct (fig. 1, x) do not appear to me to be vitellogenous or albuminiparous glands (judging, at least, from the imperfectly preserved specimen which I have examined). I regard them rather as vesiculæ seminales.As for the two appendages of the female duct, the elongated pouch is a receptaculum seminis (or poche copulatrice), and the flattened pouch (fig. 1, III) is the mucous or jelly gland. Lastly, the glandular mass of the male duct can be termed the prostate, as in other hermaphrodite Gastropods.If the presence of two genital apertures could not, in itself, demonstrate the hemaphroditism of Onchidiopsis, the structure of the genital gland is sufficient to remove all doubts. This gland is formed of parallel cæca, bifid at their extremities (fig. 3) : these terminal divisions are ovogenous, while the proximal portions of the duct are spermatogenous. As a result of this arrangement the gland appears to be composed of a superficial or external female portion, and of a deeper or central male portion. However, these two regions are not demarcated with any regularity, and in the middle portion male and female acini can be seen in sections lying side by side (fig. 2). At all events, the products of the two sexes either do not arise in the same cæcum or they do not arise in the same region of a cæcum.According to Bergh’s observations, Marsenina appears to be constructed upon the same plan,—that is to say, the genital cæca are female in their terminal portion.(3) Entoconcha.—This genus, so degraded by parasitism, has unfortunately not yet been studied by the method of serial sections. As is known, there is a female genital gland in the middle region of the body, and a number of spermatogenic capsules further back towards the posterior orifice of the body.(4) Bulloidea.—The genital gland is here formed of acini, all hermaphrodite (Bulla, Limacina, &c.); but in certain forms (Actæon, Pelta, Lobiger) I have found it composed of distinct male and female acini.(5) Aplysioidea.—The same acini produce spermatozoa and ova (Aplysia). I have found, however, that the male and female acini are distinct in certain specialised forms; the former (male) occupy the central, and the latter the peripheral region (Pneumonoderma, Clinopsis).(6) Pleurobranchoidea.—The genital gland is uniformly composed of hermaphrodite acini in Umbrella. But in Tylodina and all the Pleurobranchidæ I find that the acini are either male or female, and that the latter open into the former.(7) Nu dibranchia.—Meckel was the first to notice in certain Nudibranchs that the genital gland is formed of male and of female acini; but he supposed that these acini of different sexes were without communication with one another, and that in the genital ducts there was a male canal embedded in the female one (5).Nordmann immediately afterwards showed that in Tergipes the ovular acini open into capsules full of spermatozoa; but he took the entire hemaphrodite gland for an ovary merely, and held the sacs full of spermatozoa to be “poches de fécondation” (6).It was R. Leuckart who first determined and correctly interpreted the constitution of the hermaphrodite gland of Nudibranchs, especially Eolis (7),—i. e. peripheral acini exclusively ovular, opening into central chambers producing spermatozoa. His interpretation has been confirmed since for a greatnumber of genera by various authors (Hancock, Bergh, Trinchese,· and myself); the same is the case with those Nudibranchs in which the male and female acini are easily distinguished under a lens of low power (e. g. Fiona).It must be noticed, however, that in Eolis (Coryphella) Landsburgii the acini of the hermaphrodite gland produce ova in their distal portion and spermatozoa in their proximal portion (8) ; this arrangement has been recognised as general in all the Elysioidea (Cyerce, Hermæa, Elysia, Lima-pontia).(8) Pulmonata.—In these Gasteropods the genital gland is formed of hermaphrodite acini both in the Stylommatophora (e. g. Helix) and in the Basommatophora (e. g. Auricula).In Siphonaria (Basommatophora), according to Haller (9), each acinus of the hermaphrodite gland is exclusively of one sex, either male or female. In S. Algesiræ, which I have studied, I have observed that the conformation of the gland is precisely analogous to that found in Onchidiopsis, the Pleurobranchidæ, and the Nudibranchia,—that is to say, the peripheral female acini open into the more centrally situated male acini.However, this conformation is not so entirely different from that presented by the other Pulmonata. I know cases, in fact, in which the wall of the genital gland already shows a distinct sexual differentiation upon the two sides of the follicles, and in which the female side exhibits projections which are the rudiments of acini of this sex (Amphibola).It follows, then, from what has been said above, that examples of the various possible modes in which the genital gland is constituted are to be found side by side in all the subgroups of hermaphrodite Gastropods.(1) Ostrea.—The question of sex in oysters has long been a subject of controversy, and its solution, which presents decided difficulties, is not yet universally recognised.The first point capable of immediate demonstration is that there are both hermaphrodites and diœcious oysters.A. Hermaphrodite Oysters.a. Ostrea edulis, Linné, the hermaphroditism of which was first demonstrated by Davaine (10), and subsequently confirmed by Lacaze-Duthiers, Hock, &c.B. Ostrea stentina, Payr. (= plicata, Chemnitz); also recognised as hermaphrodite by Lacaze (11).c. Lastly, it has been recently stated that the oyster of the N.W. coast of America is also hermaphrodite; this, I suppose, is Ostrea lurida.In the genital glands of hermaphrodite oysters the spermatozoa and the ova arise in the same acini, but at different times, so that the products of the two sexes are not often to be seen in the same individual. This perfect alternation is the most striking and distinctive characteristic of the heramaphroditism of oysters ; it explains how it has been believed, and how at first sight it would still be possible to believe, that the oyster is unisexual.The male products are the first to appear. This protandry was discovered by Davaine (13), and confirmed by P. J. van Beneden (14).B. Dioecious Oysters.a. Ostrea virginica, Lister, from the E. coast of the United States (I5).b. Ostrea angulata, Lam. (= lamellosa, Broc.), the “Portuguese” oyster, from the Atlantic (16).c. Lastly, I have made out the separation of the sexes in a third species, O. cochlear, Poli, from the Mediterranean, which belongs to the same subdivision (Gryphæa) as the preceding form.I have not had the opportunity of studying this species alive, but I have had numerous specimens of it of very different sizes, collected at different times of the year.Of all these there was not a single individual which pre-sented both ova and spermatozoa in the genital gland at the same time. Each specimen had the glands either full of the products of one sex exclusively (fig. 8), or else almost empty (fig-7).Nevertheless it is impossible to suppose that they represented the successive stages, male and female, of an hermaphrodite condition : in the first place, because the individuals with male glands were no smaller than those with female glands,— there were female specimens smaller than the males, and males and females of every size; in the second place, because the appearance and conformation of the genital glands differs considerably according to the nature of the contents, as in Lamel-libranchs of different sexes. The glands with spermatozoa are formed of ramifications having an appreciably constant diameter (fig. 4, II); the glands with ova are more lobulated, and so present a distinct appearance, which shows that 0. cochlear is dioecious, like O. virginica and O. angulata.I will add a word here upon the genital apertures of 0. cochlear. They are asymmetrical, that of the left being the more anterior (fig. 6, ix), considerably in front of the adductor muscle. The “fente uro-génitale,” discovered by Hoek in 0. edulis (17) (where the genital gland opens into the urinary slit), has not in this species the simple appearance which it presents in O. edulis. The genital and renal orifices, although adjacent, are quite distinct; the genital is more anterior than the renal, less distant from the median plane (fig. 7, v), and directed outwards, whilst the renal aperture is directed towards the axis (fig. 7, x).(2) Cardium oblongum.Lacaze-Duthiers discovered the hermaphroditism of a species of this genus, C. norvegicum, from the Atlantic. I have further observed this condition in a Mediterranean species, C. oblongum, which is grouped with C. norvegicum in a special division or sub-genus (Lævicardium).In Cardum oblongum the different acini are each of one sex, either male or female; but the acini of the same sex are not confined to a particular region (fig.9) as in Onchidiopsis, for example, although the female acini open into the male acini as in the last-named genus. I have never, however, noticed ova and spermatozoa in the same cul-de-sac, as Lacaze-Duthiers found to be constantly the case in C. norvegicum (18).(3) Entovalva.The structural details of the hermaphrodite genital gland are not known. There exists a single gland, not differentiated into sexual regions, on each side (19). Probably, therefore, it is constituted like that of Ostrea and Cardium.(4) Pecten.The greater number of species of this genus hitherto examined have been found to be hermaphrodite, viz. P. glaber (20), P. Jacobæus (20a), P. maximus (21), P. opercularis (22), P. irradians and magellanicus (23).I can myself vouch for the hermaphroditism of P. glaber and maximus. Moreover I have observed that P. flexuosus, Poli, from the Mediterranean, also has the two sexes united in the same individual (fig. 10).On the other hand, among all the species examined, I have only met with two dioecious forms, viz. P. inflexus, Poli, from the Mediterranean, and P. varius, Linné, from the Atlantic, in which latter form this fact was already known (Humbert, 23a).The hermaphroditism of Pecten is recognised with unusual facility. The anterior part of the visceral mass may be readily seen from the outside to be whiter than the posterior part; the whiter part is the male region. Examination of fresh material, or a section, shows at once that the genital products of different sexes are formed in different regions of the hermaphrodite gland (fig. 10). At the same time, as is already known in the case of certain species, there is only one common genital orifice on each side (opening into the nephridium), and only a single genital duct, which ramifies in the various parts of the ovotestis (P. glaber, maximus, flexuosus). The male region is thus nearer the genital aperture than the female region.(5) Cycladidæ.A. Cyclas.—The hermaphroditism of this genus has been known from the time of v. Siebold (24). But the conformation of the genital organs has not been completely described even by subsequent authors (25) ; Stepanoff has merely made known that one portion of the gland is male and the other female.In C. cornea (fig. 12) the genital gland has a superficial position upon each side of the posterior part of the body. It is of scarcely any extent except in length (from liver to nephridium), its height and depth being inconsiderable. It is transversely divided into two portions of different appearance and size, separated by a constriction in which there is merely a ciliated duct uniting the two portions (fig. 11, in). The most anterior division is more elongated and more appreciably lobulated than the other; it extends as far as the liver. This is the male region of the gland ; the other is exclusively female (fig. 11, n).The hermaphrodite gland is thus seen to be here divided into two portions of different sexes ; and these are no longer in immediate contact (as in Pecten), but already separated from one another, though connected by the intermediation of a duct.Lastly, the posterior (female) portion of the gland is continued backwards by an hermaphrodite duct which terminates at the genital orifice. The latter, which has escaped notice hitherto) is situated outside the visceral commissure, near the most ventral point of the nephridium (where the aperture of this organ is placed), and in front of the posterior retractor muscle of the foot (fig. 12, vi).B, Pisidium (26) and Corbicula (fide v. Jhering in lit era) are also hermaphrodite. The conformation of the genital gland appears to be the same in them as in Cyclas (27).(6) Anatinacea and Poromyidæ.Several years ago (28) I made known the hermaphroditism of Tbracia, Lyonsia, Lyonsiella (Anatinacea), and of the allied family Poromyidæ (Septibranchia); and at the same time I confirmed its existence in Pandora, in which it had already been once affirmed (29). Having thus demonstrated the monoecious nature of all the genera of Anatinacea which I had examined, I was led to believe in the hermaphroditism of the entire group, a conclusion which accorded well with Lacaze’s observations upon Aspergillum (30).Since that time I have been enabled to study one other genus of this group, Clavagella, although, in spite of numerous efforts, I have not succeeded in obtaining either Anatina or Pholadomya, which appear to be rare. I have observed the same monœcious arrangement in Clavagella as in the other Anatinacea previously studied. There can accordingly be no doubt as to the hermaphroditism of this group, since no genus belonging to it has yet been found to be diœcious.Like all the Anatinacea, Clavagella possesses two testes, two ovaries, and four distinct genital apertures. The two testes are symmetrically placed in the pedal projection (fig. 15, VIII), i.e. in the ventral part of the body; the two ovaries, which are much more voluminous, compose almost the entire posterior visceral mass as far as the nephridia (figs. 15 and 16).The genital pores are situated behind ; those of the testes on the sides of the base of the foot, ventrally or internally to the visceral commissure (fig. 14, VII); those of the ovaries a little further back, dorsally or externally to the same commissure (fig. 14, IV).If any other group of hermaphrodite animals is taken into consideration, one is struck by the general uniformity in the structure of the genital glands. In the Mollusca, on the other hand, as has been shown above, there is a very great diversity in the structure of the hermaphrodite gland.Indeed, it was Leuckart who was the first to recognise that among the Euthyneurous (or hermaphrodite) Gastropods the genital gland is not always formed in the same manner, and that different types of structure are recognisable in it. The facts known to us to-day show that such is the case not only for the Gastropoda Euthyneura, but also for all the remaining groups of hermaphrodite molluscs—the Amphineura, Gastropoda Streptoneura, and Lamellibranchia.Four different principal types can be distinguished in the conformation of the hermaphrodite gland of molluscs, but certain forms are transitional between one type and another.1. The undifferentiated gland, i. e. with acini completely hermaphrodite.—As development shows (vide infra), the most simple condition of an hermaphrodite genital gland is that in which the gonadial wall is still undifferentiated, and produces ova and spermatozoa side by side. This condition is exhibited in—(1) Valvata.(2) A great number of Tectibranchs (e. g. Bulla, Aplysia, Umbrella).(3) Almost the entire group of Pulmonata.(4) Ostrea edulis and stentina.A commencement of specialisation is observable in Neo-meniidæ, where each genital gland generally gives rise to male products towards its lateral face, and to ova towards its axial face.Amphibola among Pulmonates (vide supra) also furnishes a transition to the following condition.2. Gland with separate male and female acini (but as yet without separation into different male and female regions).—After the undifferentiated condition comes that in which the gonadial surface shows a clearly marked specialisation into distinct male and female acini,—the acini, however, not forming regions of different sex.This arrangement exists in—(1) Various Tectibranchs (e. g. Lobiger, Pelta).(2) Pleurobranchidæ, Tylodina, and Nudibranchia (except Elysioidea).(3) Siphonaria.(4) Cardium oblongum.It must be noticed here that the female acini open into the male acini, and that the acini of the same sex tend to group themselves together, as may already be observed in various Nudibranchs (where the female acini are the most “eccentric,” i. e. superficial). This arrangement is clearly marked, and furnishes in Onchidiopsis and Pneumonoderma (vide supra) a transition to the next condition.3. Gland with separate male and female regions (with a common duct).—The type which now presents itself is that in which the acini of the same sex are all united together in such a way as to constitute in the hermaphrodite gland a male and a female part distinct from one another, these two parts having, nevertheless, a single genital aperture and a common duct.This conformation is characteristic of—(1) The hermaphrodite species of the genus Pecten, where the male and female regions are contiguous and form an undivided hermaphrodite gland (fig. 10).(2) The Cycladidæ, where it is already observable that the two regions are fairly separated, and only connected by their duct (fig. 12), which forms a transition to the next type.4. Male and female glands in the same individual entirely distinct from one another, and with special ducts.—The acini of each sex can form a region absolutely separate from that constituted by the acini of the other sex, these two regions each having their special duct, and so forming veritable testis and ovary.At the same time the vas deferens and the oviduct may open into a common orifice (Poromyidæ), or there may be no common orifice for the two glands. This last condition, which represents the highest specialisation of the hermaphrodite genital gland, is only found in the Anatinacea among Lamelli-Branchs, and in Entoconcha among Gastropods.The hermaphroditism of molluscs is not, so to say, selfsufficient; in other words, the eggs of one individual have to be fertilised by the spermatozoa of another.Speaking generally, the two kinds of products are not ripe at the same moment in the same individual: an interval of greater or less duration separates the two periods of maturity of the different sexual elements.Leuckart was the first to make known that in certain Opis-thobranchiate Gastropods the period of male maturity precedes that of female maturity (31), i. e. the hermaphroditism of these forms is protandric.This protandry ought to be regarded as a general phenomenon in Euthyneurous Gastropods. This is notoriously the case in the Pulmonates (32), and it has been recognised in the various Opisthobranchs which have been studied from this point of view, viz. Lobiger (33), the Thecosomatous “Ptero-pods,” e.g. Clio striata (34), Cymbulia (35), Desmopterus (36) ; Nudibranchs, among which I have observed it in Eolis and Elysia; and lastly I may add Clione limacina (Gym-nosomata), in which I have noticed that individuals of a length of 15 millimetres (or less) do not as yet show any ova in their genital gland, but stages in the development of spermatozoa only.Similarly in Entoconcha testicular capsules are only to be observed in individuals in which the eggs are not yet well developed (37).For Neomenia, among Amphineura, protandry is equally probable (38).In Lamellibranchs no general statement can be made with certainty. I have been unable to examine young individualsof Anatinacea, or hatched specimens of Cyclascornea smaller than 4 millimetres : at this size both spermatozoa and ova are already to be found, and not infrequently even developing eggs among the gills. But according to the old observations of Davaine (39) and P. J. van Beneden (40), Ostrea edulis is protandric.The alternating activity of the two sexes in the same hermaphrodite Molluscan individual is a perfectly certain fact, and explains various mistakes like that of Saint-Loup, who believed he had found a unisexual organisation in Aplysia associated with external sexual characteristics (41).It must, however, be remarked that this succession of the two sexual conditions may be more or less rapid according to the particular genera under consideration, and that, consequently, the alternation is better marked in certain organisms than in others. It seems to me to be more appreciable where the hermaphroditism of the genital gland is most complete (i. e. where the ova and spermatozoa are produced at the same spot, as in Ostrea, Aplysia, &c.) than where there are acini or regions of different sex (Nudibranchs, &c.).As for the general fact that the male precedes the female activity, even in the absence of physiological observations it might have been deduced from the morphological constitution of the genital glands in the adult of a great number of hermaphrodite molluscs. In those forms, in fact, which possess acini or regions of each sex, the male part is always nearest the efferent ducts or the genital aperture : the male products will accordingly be the first to reach them. This arrangement exists in Pecten, Nudibranchs, Pleurobranchidæ, Onchidiopsis, Entoconcha, Pneumonoderma, Clionopsis, &c. Cyclas alone constitutes an exception,—a fact for which I can offer no explanation.For the rest, our knowledge of the physiological evolution of individual hermaphroditism in other groups of the animal kingdoms shows that protandry is general in all forms which have been examined from this point of view, e. g. Sponges, Plathelminthes (Trematodes and Cestodes), hermaphrodite Nematodes (Pelodytes, Rhabdonema nigrovenosa), Myzostomidæ, parasitic Isopods, Ascidians and Salps, Myxine, Chrysophris, &c.According to the theory of the sexuality of the embryonic layers, the female reproductive elements should be of endo-dermic, and the male elements of ectodermic origin. As regards the Mollusca this view has been maintained by Fol (“Pteropoda “Thecosomata, 42).But this theory has not been confirmed for any other mollusc, even for any of the nearest allies of the “Pteropods,” viz. the Tectibranchs (e. g. Apiysia).I have not had an opportunity of studying the fresh larvæ of Thecosomatous “Pteropods;” but in sections of various preserved larvæ I have been able to make out that the “corps pyriforme” (which, according to Fol, is the testicular part of the future hermaphrodite gland) neither has the structure of a testicle nor contributes at all to the formation of the genital gland. The latter is unique from its origin, as it is in its final condition. The same is the case in the other molluscs hitherto studied.However, according to Trinchese (43), in Bosellia (a near ally of Elysia) the ova are not produced in the same part of the body as the spermatozoa, although this author is unable to explain how the male and female parts of the hermaphrodite acini eventually combine with one another. Now in Elysia, specimens of which I have examined at every age from this point of view, I find that the first ova arise in the same acini in which spermatozoa alone were present in the stage immediately preceding. So that it is impossible in this case to hold that the hermaphrodite gland results from the fusion of two male and female parts of independent origin. The same fact may be observed equally well in the “Pteropod” Clione (vide supra).On the other hand, in a form where the male and female acini are sharply separated in the adult state, viz. Pelta (=Runcina), I find that in the young individual ova and spermatozoa arise side by side on the walls of a common pouch.I conclude from this that the undifferentiated condition of the hermaphrodite gland is the most primitive, and the morphological observations recorded above (II) lead also to the same conclusion. The Nudibranchs and Pleurobranchidæ (with acini either male or female) are more specialised than Umbrella (with hermaphrodite acini); Onchidiopsis (with acini either male or female) is more specialised than Vaivata (with hermaphrodite acini); Pneumonoderma (with acini either male or female) is more specialised than Aplysia (with hermaphrodite acini), &c.; lastly, those Lamellibranchs with male and female glands altogether separated (Anatinacea) are the most specialised (cf. the closure of the mantle, the reduction of the foot, the complexity of the gill, involving the loss of the external layer of the external branchial lamella).In direct opposition to the theory of the sexuality of the embryonic layers, we find that the hermaphrodite gland has a single origin in the mesoderm (which is itself endodermic), not only in the Opisthobranchs and Pulmonates hitherto studied—where the gland is not yet divided into regions of different sex,—but also in Cyclas (44), where two distinct male and female regions exist (fig. 12, v and xiv). The same mesodermic origin is, moreover, to be observed in the case of both the male and female glands of those forms with separate sexes, such as Chiton, Paludina, and the Cephalopods. These results are also in complete accord with what one finds in other groups of hermaphrodite Invertebrates, e. g. Oligochæta (45), Sagitta, Turbellaria, Plathelminthes, Hirudinea (46).Lastly, the fact that the wall of the genital gland is in continuity with the coelomic epithelium (mesoderm) in Nuculidæ (Lamellibranchs), Neomeniidæ (Amphineura), and Cephalopoda, shows that the genital gland, whether male, female, orhermaphrodite, is an organ possessing a single mesodermic mode of origin, as in all the other Triploblastica.It has just been shown that, both from the phylogenetic and ontogenetic points of view, the hermaphrodite condition with separate male and female glands is the most specialised, and the undifferentiated hermaphrodite condition (with gland producing spermatozoa and ova at the same spot) the most archaic. Now this last state is that which most nearly approximates to the unisexual condition, since it only differs in the supplementary production of the elements of the other sex,—a phenomenon which is sometimes to be observed as an abnormality in dioecious molluscs (e. g. Anodonta and Ampullaria).The question may, then, be asked in the case of the Mollusca,1. Whether the hermaphrodite state is not derived from the unisexual; and, in the event of an affirmative reply,2. Upon which of the sexes the hermaphrodite condition has become established.In the following pages I shall try to show—1. That hermaphroditism is not a primitive arrangement in the Molluscan phylum (47), and that it has been derived from the unisexual state.2. That it has become superimposed upon the female condition.1. Hermaphroditism has been derived from the unisexual condition.—Let us consider separately the classes in which hermaphroditism is found and those in which the diœcious state alone exists.i. In the classes in w