Reproduction in the Mollusca invariably takes place by means of eggs, which, after being developed in the ovary of the female, are fertilised by the spermatozoa of the male. As a rule, the eggs are ‘laid,’ and undergo their subsequent development apart from the parent. This rule, however, has its exceptions, both among univalve and bivalve Mollusca, a certain number of which hatch their young from the egg before expelling them. Such ovoviviparous genera are Melania, Paludina, Balea, and Coeliaxis among land and fresh-water Mollusca, and Cymba and many Littorina amongst marine. The young of Melania tuberculata, in Algeria, have been noticed to return, as if for shelter, to the branchial cavity of the mother, some days after first quitting it. Isolated species among Pulmonata are known to be ovoviviparous, e.g. Patula Cooperi, P. Hemphilli, and P. rupestris, Acanthinula harpa, Microphysa vortex, Pupa cylindracea and muscorum, Clausilia ventricosa, Opeas dominicensis, Rhytida inaequalis, etc. All fresh-water Pelecypoda yet examined, except Dreissensia, are ovoviviparous.

The number of eggs varies greatly, being highest in the Pelecypoda. In Ostrea edulis it has been estimated at from 300,000 to 60,000,000; in Anodonta from 14,000 to 20,000; in Unio pictorum 200,000. The eggs of Doris are reckoned at from 80,000 to 600,000, of Loligo and Sepia at about 30,000 to 40,000. Pulmonata lay comparatively few eggs. Arion ater has been observed to lay 477 in forty-eight days (p. 42). Nests of Helix aspersa have been noticed, in which the number of eggs varied from about 40 to 100. They are laid in little cup-shaped hollows at the roots of grass, with a little loose earth spread over them. The eggs of Testacella are rather large, and very elastic; if dropped on a stone floor they will rebound sharply several inches. The Cochlostyla of the Philippines lay their eggs at the tops of the great forest trees, folding a leaf together to serve as a protection.

The eggs of the great tropical Bulimus and Achatina, together with those of the Macroön group of Helix (Helicophanta, Acavus, Panda) are exceedingly large, and the number laid must be decidedly less than in the smaller Pulmonata. Bulimus oblongus, for instance, from Barbados, lays an egg about the size of a sparrow’s (Fig. 38), Achatina sinistrorsa as large as a pigeon’s. The Cingalese Helix Waltoni when first hatched is about the size of a full-grown H. hortensis. There is, in the British Museum, a specimen of the egg of a Bulimus from S. America (probably maximus or popelairanus) which measures exactly 1¾ inch in length.

The Limnaeidae deposit their eggs in irregular gelatinous masses on the under side of the leaves of water-plants, and on all kinds of débris.

The Rachiglossa or marine carnivorous families lay their eggs in tough leathery or bladdery capsules, which are frequently joined together in shapes which differ with the genus. Each capsule contains a varying number of ova. The cluster of egg-capsules of Buccinum undatum is a familiar object on all our sandy coasts. The capsules of Purpura lapillus are like delicate pink grains of rice, set on tiny stalks. They are not attached to one another, but are set closely together in groups in sheltered nooks of the rocks. A single Purpura has been observed to produce 245 capsules! Busycon lays disc-shaped capsules which are all attached at a point in the edge to a cartilaginous band nearly 3 feet in length, looking like a number of coins tied to a string at equal distances from one another. In Murex erinaceus the egg-capsules are triangular, with a short stalk. They are deposited separately in clusters of from 15 to 150, there being about 20 ova in each capsule. It appears that all the species of the same genus have by no means the same method of depositing their eggs, nor do they always produce eggs of at all similar size or shape. Thus, of two British species of Nassa, N. reticulata lays egg-capsules in shape like flattened pouches with a short stalk, and fastens them in rows to the leaves of Zostera; M. incrassata, on the other hand, deposits solitary capsules, which are shaped like rounded oil-flasks. Neptunea antiqua lays its eggs in bunched capsules, like Bucc. undatum (Fig. 40), but the capsules of N. gracilis are solitary.

In Natica the eggs are deposited in what looks like a thick piece of sand-paper, curled in a spiral form (Fig. 41). The sand is agglutinated by copious mucus into a sort of sheet, and the eggs are let into this, sometimes (N. heros) in regular quincunx form. Ianthina attaches its eggs to the under side of its float (Fig. 42). The Trochidae deposit their eggs on the under side of stones and sea-weeds, each ovum being contained in a separate capsule, and all the capsules glued together into an irregular mass of varying size. The female of Galerus chinensis hatches her eggs by keeping them between her foot and the stone she adheres to. They are laid in from 6 to 10 capsules, connected by a pedicle and arranged like the petals of a rose, with 10 to 12 eggs in each capsule. Those Littorina which are not ovoviviparous deposit their spawn on sea-weeds, rocks, and stones. The eggs are enveloped in a glairy mass which is just firm enough to retain its shape in the water; each egg has its own globule of jelly and is separated from the others by a very thin transparent membrane.[242]

Chiton marginatus, when kept in captivity, has been noticed[243] to elevate the posterior part of the girdle, and to pour out a continuous stream of flaky white matter like a fleecy cloud, which proved to be of a glutinous nature. It then discharged ova, at the rate of one or two every second, for at least fifteen minutes, making a total of 1300 to 1500, each being about ¹/₁₀₀ inch diameter. The ova were shot into the glutinous cloud, which seemed to serve as a sort of nidus to entangle the ova and prevent them being carried away. The subsequent development was rapid, and in seven days the young Chiton was hatched, being then about ¹/₂₀ inch long. Lovén has described the same species as laying its eggs, loosely united in clusters of seven to sixteen, upon small stones. There is probably some mistake about the identification, but the observation illustrates the varying methods of oviposition among allied forms.

Not very much is known with regard to the ovipositing of the Cephalopoda, especially those which inhabit deep water. Masses of ova arranged in very various forms have occasionally been met with floating in the ocean, but it is next to impossible to determine to what species, or even genus, they belong.[244]

In Loligo punctata the ova are contained in small cylindrical cases measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each case. Hundreds of these cases are attached together like a bundle of sausages or young carrots, and the movements of the embryos within can be distinctly noted. Sepia officinalis lays large black pear-shaped capsules, each of which is tied to some place of attachment by a kind of ribbon at the upper end of the capsule, the whole forming a large group like a bunch of grapes. Octopus vulgaris deposits thousands of small berry-shaped ova, attached to a string which runs along the centre of the mass (Fig. 43).

The so-called shell of the female Argonauta is nothing more than a form of protection for the ova, and is in no sense homologous to the ordinary molluscan shell. The ova consist of a large granulated mass, attached to a many branched stem; they are contained in the spire of the shell, in contact with the posterior part of the body of the mother, but sometimes project externally beyond the coil of the spire.

Certain species possess the curious property of laying their eggs on the outside of their own shells. Buccinopsis Dalei is not unfrequently found decorated with its own egg-capsules. Possibly this species, which lives on oozy ground, finds this the only secure place of attachment for its progeny. Neritina fluviatilis has a similar habit, and so have many other species of Neritina and Navicella. It is not quite clear, in the latter cases, whether the eggs are laid by the specimens on whose shell they are found, or whether they are deposited by others. In either case, perhaps the shell is the safest place for them in the rapid streams which both genera frequent. Specimens of Hydrobia ulvae taken on the wet sands at the mouth of the Dee, are found to have several little rounded excrescences scattered over the surface of the shell. These, on examination, are found to be little masses of small sand-grains, in the centre of which is a clear jelly containing segmenting ova or young embryos. Here again, in all probability, the shell is the only comparatively stable object, in the expanse of shifting sands, on which the eggs can be laid.[245]

The pulmonate genus Libera, which occurs on a few of the island groups in the Central Pacific, is remarkable for the habit of laying its eggs within its own cavernous umbilicus, which is narrowed at the lower part. The eggs number from four to six, or the same number of very young shells may be seen closely packed in the cavity, each being in shape exactly like a young Planorbis. This constriction of the umbilicus does not occur till the formation of the last two whorls, i.e. till the animal is sexually mature. Some species, but not all, provide for the safety of their eggs more completely by forming a very thin shelly plate, which nearly closes the umbilical region, and breaks away or is absorbed to facilitate the escape of the young shells.[246]

Union of Limax.—With regard to the act of union itself, the method in certain species of Limax deserves special notice. L. maximus has been observed at midnight to ascend a wall or some perpendicular surface. A pair then crawl round and round one another emitting a quantity of mucus which at length forms a patch, 2 to 2½ inches in diameter. When this acquires consistency the pair begin to twist round each other in corkscrew form, and detach themselves from the wall, hanging by a cord of the thickened mucus, about 8–15 inches long, and still twisting round each other. The external generative organs are then protruded and copulation takes place, after which the bodies untwist, separate, and crawl up the cord again to the wall.[247]

Periodicity in Breeding.—In the marine Mollusca, the winter months appear to be the usual time for the deposition of eggs. Careful observations have been made on the Mollusca occurring at Naples,[248] and the general result seems to be that for all Orders alike the six winter months from November to April, roughly speaking, are the breeding time. Scarcely any forms appear to breed habitually in August, September, or October. On our own coasts, Nudibranchiata come in shore to deposit their ova from January to April. Purpura lapillus may be observed depositing ova all the year round, but is most active from January to April. Buccinum undatum breeds from October to May; Littorina all the year round.

The land Mollusca exhibit rather more periodicity than the marine. In temperate climates they breed exclusively in the summer months. In the tropics their periods are determined by the dry and rainy seasons, where such occur, otherwise they cohabit all the year round. According to Karl Semper, the snails of the warm Mediterranean region arrive at sexual maturity when they are six months old, i.e. before they are fully grown. After a rest of about three months during the heat of summer, a second period of ovipositing occurs.[249] Helix hortensis and H. nemoralis ascend trees, sometimes to a height of forty feet, when pairing.[250]

Hybridism as the result of union between different species of Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at Anduze (Gard) as many as twenty specimens of Pupa cinerea united with Clausilia papillaris. No offspring seem to have resulted from what the professor calls ‘this innocent error,’ for the wall was carefully scrutinised for a long time, and no hybrid forms were ever detected.

The same observer noticed, in the Luxembourg garden at Paris, and M. Gassies has noticed[252] at various occasions, union between Helix aspersa and nemoralis, H. aspersa and vermiculata, between Stenogyra decollata and a Helix (sp. not mentioned), H. variabilis and pisana, H. nemoralis and hortensis. In the two latter cases a hybrid progeny was the result. It has been noticed that these unions generally took place when the air was in a very electric condition, and rain had fallen, or was about to fall, abundantly.

Of marine species Littorina rudis has been noticed[253] in union both with L. obtusata and with L. littorea, but no definite facts are known as to the result of such unions.

Self-impregnation (see p. 44).

Development of the Fertilised Ovum.—The first stages in the development of the Mollusca are identical with those which occur in other classes of animals. The fertilised ovum consists of a vitellus or yolk, which is surrounded with albumen, and is either contained in a separate capsule, or else several, sometimes many, ova are found in the same capsule, only a small proportion of which ultimately develop. The germinal vesicle, which is situated at one side of the vitellus, undergoes unequal segmentation, the result of which is usually the formation of a layer of small ectoderm cells overlying a few much larger cells which contain nearly the whole of the yolk. The large cells are then invaginated, or are simply covered by the growth of the ectoderm cells. The result in either case is the formation of an area, the blastopore, where the inner cells are not covered by the ectoderm. The blastopore gradually narrows to a circular opening, which, in the great majority of cases, eventually becomes the mouth. The usual differentiation of germinal layers takes place, the epiblast eventually giving rise to the epidermis, nervous system, and special sense organs, the hypoblast to the liver and to the middle region of the alimentary tract, the mesoblast to the muscles, the body cavity, the vascular, the excretory and reproductive systems. The next, or trochosphere (trochophora) stage, involves the formation of a circlet of praeoral cilia, dividing the still nearly spherical embryo into two unequal portions, the smaller of which consists simply of the prostomium, or part in front of the mouth, the larger bearing the mouth and anus.

So far the series of changes undergone by the embryo are not peculiar to the Mollusca; we now come to those which are definitely characteristic of that group. The stage next succeeding the development of the trochosphere is the definitive formation of the velum, a process especially characteristic of the Gasteropoda and Pelecypoda, but apparently not occurring in the great majority of land Pulmonata.

The circlet of cilia becomes pushed more and more towards the anterior portion of the embryo, the cilia themselves become longer, while the portion of the body from which they spring becomes elevated into a ridge or ring, which, as a rule, develops on each side a more or less pronounced lobe. The name velum is applied to this entire process of ciliated ring and lobes, and to the area which they enclose.

In this so-called veliger stage, the velum serves, in the first place, to cause rotation of the larva within the egg-capsules, and, after hatching, as an organ of locomotion. As a rule, the velum disappears entirely in the adult mollusc after the free-swimming stage is over, but in the common Limnaea stagnalis it persists, losing its cilia, as the very prominent circum-oral lobes. Simultaneously with the development of the velum, and in some cases earlier, appear the rudiments of the shell-gland and of the foot, the latter being situated on the ventral side, between the mouth and anus, the former on the dorsal side, behind the velum, and above the surface of the eventual visceral sac. Thus the prime characteristics of the veliger stage, subsequent to the appearance of the velum itself, are the development of the visceral sac and shell-gland on the upper, and of the foot on the under side. According to Lankester the primitive shell-gland does not, as a rule, directly give rise to the shell of the adult mollusc, but becomes filled up by a horny substance, and eventually disappears; the permanent shell then forms over the surface of the visceral hump from the original centre of the shell-gland. It is only in Chiton, and possibly in Limax, that the primitive shell-sac is retained and developed into the final shell-forming area, which is much wider, and extends to the edges of the mantle. Within the velar area first appear the rudiments of the tentacles and eyes; when these become developed the velum atrophies and disappears.

Several of these veligers when captured in the open sea have been mistaken for perfect forms, and have been described as such. Thus the larva of Dolium has been described as Macgillivrayia, that of a Purpura as Chelotropis and Sinusigera, that of Aporrhais pes pelecani as Chiropteron, that of Marsenia conspicua as Brownia, Echinospira, and Calcarella.

Cephalopoda.—The embryonic development of the Cephalopoda is entirely distinct from that of all other Mollusca. The segmentation of the vitellus is partial, and the embryo is furnished with a vitelline sac, which is very large in the majority of cases (Fig. 48). There is no free-swimming stage, but the embryo emerges from the egg fully developed.

Differences of Sex.—In the Mollusca there are two main types of sexual difference: (i) sexes separate (dioecious type), (ii) sexes united in the same individual (hermaphrodite type).

In some cases—e.g. certain Pelecypoda—what is practically a third type occurs. The animal is hermaphrodite, but the male and female elements are not developed simultaneously, i.e. the same individual is at one time female, at another male.

1. The sexes are separate in

• All Cephalopoda.
• Gasteropoda Amphineura (except Neomeniidae).
• Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia).
• Scaphopoda.
• Many Pelecypoda.

2. The sexes are united in

• Gasteropoda Opisthobranchiata.
• Gasteropoda Pulmonata.
• Certain Pelecypoda.[254]

In the dioecious Mollusca, sexual union is the rule, but is by no means universal. In some instances,—e.g. Vermetus, Magilus, Patella, Haliotis, Crepidula, Chiton, the Scaphopoda—the form and habits of the animal do not admit of it; in others (many Trochus) a male copulative organ is wanting. When this is the case, the male scatters the spermatozoa freely; the majority must perish, but some will be carried by currents in the direction of the female.

When the sexes are separate, the female is frequently larger than the male. This is markedly the case in Littorina, Buccinum, and all the Cephalopoda; in Argonauta the difference is extreme, the male not being more than ¼ the size of the female.

Those hermaphrodite Mollusca which are capable of sexual union (Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently divided into two sections, according as (1) there are separate orifices for the male and female organs, or (2) one orifice serves for both. To the former section (Digonopora[2]) belong the Limnaeidae, Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including all the Pteropoda; to the latter (Monogonopora[255]) nearly all the Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata. In the latter case during union, mutual impregnation takes place, and each of the two individuals concerned has been observed (compare p. 42) to deposit eggs. In the former, however, no such reciprocal act can take place, but the same individual can play the part of male to one and female to another, and we sometimes find a string of Limnaea thus united, each being at once male and female to its two adjacent neighbours.

The Reproductive System.—Broadly speaking, the complicated arrangements which are found in Mollusca resolve themselves into modifications of three important factors:—

(a) The gonads or germ-glands, in which are developed the ova and the spermatozoa. These glands are generally known as the ovary in the female, the sperm-gland or testis in the male.

(b) The channels which provide for the passage of the seminal products; namely, the oviduct in the female, the vas deferens or sperm-duct in the male.

(c) The external generative organs.

Dioecious Mollusca.—The common Littorina obtusata will serve as a typical instance of a dioecious prosobranchiate, exhibiting the simplest form of organs. In the female the ovary, a lobe-shaped body, is embedded in the liver. An oviduct with many convolutions conveys the ova into the uterus, an oblong chamber which consists simply of a dilatation of the oviduct. The ova descend into the uterus, which is sometimes furnished with a seminal pouch. In this seminal pouch, or above it, in the oviduct, the ova come into contact with the spermatozoa. The lower part of the uterus secretes a gelatinous medium (or capsule, as the case may be) in which the fertilised ova become enclosed previous to exclusion. In position the oviduct abuts on the kidney, while the uterus is in close proximity to the rectum, and the female external orifice is found close to the anus, within the branchial cavity.

The male organs of Littorina are more simple. The testis is lodged, like the ovary, in the liver; the vas deferens is, like the oviduct, convoluted, and eventually traverses the right side of the neck, emerging near the right tentacle, and terminating in the penis or external copulative organ (Fig. 50).

This system prevails, with but slight modifications in detail, throughout the prosobranchiate Gasteropoda. The most important modification is the passage of the seminal products in certain cases (many of the Diotocardia) through the right kidney, with which the oviduct and vas deferens always stand in close relation. The same arrangement occurs in the Scaphoda and some Pelecypoda.

The penis varies greatly in form and size. In the Strombidae (see Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in Littorina it is somewhat spinulose at one side; in Paludina a portion of it is lodged in the right tentacle, which becomes atrophied and much more obtuse than the tentacle on the left side.

Spermatozoa.—The shape of the spermatozoa and of the ova in Mollusca is of the usual type. In Paludina Ampullaria, and certain species of Murex two types of spermatozoa occur, one hair-like, the other worm-like, three times as long as the former, and not tapering at one end. The former type alone take part in fertilisation, and penetrate the ovum. It has been suggested that these worm-like spermatozoa are a kind of incipient ova, and indicate a possible stage in commencing hermaphroditism. And, since the nearest allies of the Prosobranchiata (in which these types occur) are hermaphrodite (i.e. the Opisthobranchiata and Pulmonata), it is not unreasonable to suppose that the Prosobranchiata should show some tendency towards hermaphroditism in their genital glands.[256]

Cephalopoda.—The special characteristic of the reproductive organs in female Cephalopoda is the development of various glands, some of considerable size, in connexion with the ovary and oviduct. Sepia, Loligo, and Sepiola are furnished with two large nidamental glands, which open into the mantle cavity independently of the oviduct. Their purpose is to produce a viscid mucus, which envelops the ova at the moment of their emission and eventually hardens into the egg-capsules. A pair of accessory nidamental glands occur in Sepia, as well as a pair of smaller glands situated on the oviduct itself.

In many of the male Cephalopoda the vas deferens is long and dilated at its outer end into a glandular reservoir, within which are formed the spermatophores, or narrow cylindrical packets which contain a very large number of spermatozoa. When charged, the spermatophores pass into what is known as Needham’s sac, where they remain until required for use. These spermatophores are a very characteristic part of the reproductive arrangements in the Cephalopoda. The male of Sepia has been noticed to deposit them, during union, upon the buccal membrane of the female. During the emission of the ova by the female, the spermatophores, apparently through the agency of a kind of spring contained at one end, burst, and scatter the spermatozoa over the ova.

The Hectocotylus Arm.—Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called hectocotylus of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement.

The hectocotylus is confined to the dibranchiate Cephalopoda, and its typical form, i.e. when part of the arm becomes disengaged and left with the female, occurs only in three genera of the Octopodidae, viz.[Argonauta, Ocythoe (Philonexis), and Tremoctopus. In all of these, the male is many sizes smaller than the female. In Argonauta the third arm on the left side becomes hectocotylised. At first it is entirely enveloped in a kind of cyst, in such a way that only a small portion of the tip projects; subsequently the cyst parts asunder, and allows the arm to become expanded to its full length, which considerably exceeds that of the other arms. At a certain point the acetabula or suckers terminate, and the remainder of the arm consists of a very long, tapering, sometimes thread-like filament, which is pointed at the extreme tip. It is not yet known how the spermatophores find their way into the hectocotylus, or how the hectocotylus impregnates the ova of the female. The arm thus affected is not always the same. In Tremoctopus it is the third of the right side, in the Decapoda the modification usually affects the fourth of the left.

This singular property of the male Cephalopoda has only recently been satisfactorily explained. It is true that Aristotle, more than twenty-two centuries ago, distinctly stated that certain of the arms were modified for sexual purposes. Speaking of what he calls the polypus (which appears to represent the Octopus vulgaris of the Mediterranean), he says: ‘It differs from the female in having what the fishermen call the white sexual organ on its arm;’ again, ‘Some say that the male has something of a sexual nature (αὶδοιῶδές τι) on one of its arms, that on which the largest suckers occur; that this is a kind of muscular appendage attached to the middle of the arm, and that it is entirely introduced within the funnel of the female’. Unfortunately the word translated by introduced is corrupt, and can only be restored conjecturally. He again remarks, ‘The last of the arms, which tapers to a fine point and is the only whitish arm, it uses in sexual union.’[257]

The typical hectocotylus seems to have entirely escaped notice until early in the present century, when both Delle Chiaje and Cuvier described it, as detected within the female, as a parasite, the latter under the name of Hectocotylus octopodis. Kölliker, in 1845–49, regarded the Hectocotylus of Tremoctopus as the entire male animal, and went so far as to discern in it an intestine, heart, and reproductive system. It was not until 1851 that the investigations of Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H. Müller, in 1853, completed the discovery by describing the entire male of Argonauta.

In all genera of dibranchiate Cephalopoda except Argonauta, Ocythoe, and Tremoctopus, one of the arms is sexually modified in various ways, but never becomes so much prolonged, and is never detached and left with the female. In Loligo Forbesii Stp. the fourth arm on the left has 23 pairs of regularly developed acetabula, which then lessen in size and disappear, being replaced by long pedunculated papillae, of which there are about 40 pairs. In Loligo vulgaris Lam. and L. Pleii Orb. 18 or 19 pairs of acetabula are regularly formed, and then occur 40 pairs of papillae, as in Forbesii. In other species of Loligo (gahi Orb., brevis Bl., brasiliensis Orb.) only the outer row of suckers becomes modified into papillae after about the 20th to the 22nd pair. In Sepioteuthis sepioides the modification is the same as in the Loligo last mentioned, but the corresponding arm on the right side is so covered with acetabula towards its extreme end, that it is thought that it in some way co-operates with the hectocotylised left arm.

In Octopus, the third arm on the right side is subject to modification. This arm is always shorter than the corresponding arm on the other side, and carries fewer suckers, but is furnished at the extreme tip with a peculiar kind of plate, which connects with the membrane at the base of the arm by a channel of skin, which probably conveys the spermatophores up to the tip.

In Octopus vulgaris, the species referred to by Aristotle, the hectocotylised arm is short, thin in its outer half and pointed at the extremity, while the fold of skin is very white, and gives the arm an appearance of being divided by a cleft at the side. At the same time, an unusual development of one or two suckers on the arm is not uncommon.[259]

It is believed that in the Tetrabranchiate Cephalopoda (Nautilus) a union of the four inner ventral arms may correspond functionally to the hectocotylising of the arm in the Dibranchiates.

Hermaphrodite Mollusca.—(a) Monogonopora.—The reproductive system in the hermaphrodite Mollusca is far more complicated than in the dioecious, from the union of the male and female organs in the same individual. As a type of the Monogonopora, in which a single orifice serves for both male and female organs, may be taken the common garden snail (Helix aspersa), the accompanying figure of which is drawn from two specimens found in the act of union (Fig. 53).

Beginning from the inside and proceeding outwards we have firstly the hermaphrodite gland or ovo-testis (H.G.), a yellowish white mass of irregular shape, embedded in the liver (L.) and forming part of its spiral but not reaching quite to the apex. Within this gland are developed the ova and spermatozoa. The former are rather large round cells, produced within the outer wall of the gland, while the spermatozoa, which are produced in the more central part, are thread-like bodies, generally aggregated in small bundles. From the hermaphrodite gland the ova and spermatozoa pass through the upper part of the hermaphrodite duct (H.D.), which is always more or less convoluted. Below the convoluted portion, the duct opens into the albumen gland (A.G.), a large linguiform mass of tissue which becomes dilated at the time of pairing, and secretes a thick viscid fluid which probably serves to envelop the ova. Up to this point both the male and female elements follow the same course, but on their exit from the albumen gland they diverge. The hermaphrodite duct becomes greatly enlarged, and is partially divided by a kind of septum into a male and female portion. These run parallel to one another, the larger or female portion (H.DF.), through which the ova pass (and which is sometimes termed the uterus) being dilated into a number of puckered folds, while the smaller or male portion (H.DM.) is comparatively narrow, and not dilated. At their anterior end, the two portions of the duct separate completely from one another, the female portion being then termed the oviduct (OV.) and the male portion the vas deferens (V.D.).

Following first the oviduct, we find that it soon widens into the vagina (V.), which is furnished with a pair of mucous glands (M.G.), one on each side. These are much branched, and resemble little bunches of whitish seaweed. A little above the mucous glands a long tube diverges from the vagina, which is furnished with a produced coecum (C.) and a pouch, the spermatheca (SP.) at the extreme end. In this pouch, and in the duct leading to it, is stored the spermatophore received in union with another snail. Just below the mucous glands the vagina is joined by the dart sac (D.S.), which is more fully described below. Finally, at its lower end the vagina unites with the penis sac at a point just posterior to the common orifice.

Returning now to the male organs, we find that the vas deferens is the continuation of the male portion of the hermaphrodite duct, after its final separation from the female portion. It passes under the retractor muscle of the upper right tentacle, which has been cut away in the specimen figured, to dissect it out. Just before the vas deferens widens into the penis sac, it branches off into a long and tapering tube, the flagellum, in which the spermatozoa are stored and become massed together in the long packet known as the spermatophore. The penis sac (P.S.) is the continuation of the vas deferens beyond the point at which the flagellum diverges. It joins the vagina at its extreme anterior end, uniting with it to form the common genital aperture, which cannot be exactly represented in the figure. The penis itself lies in the interior of the penis sac, and is a rather long muscular tube which is protruded during union, but at other times remains retracted within the sac.

In the Helicidae generally, the form of the generative organs varies with each separate species, sometimes merely as regards the size of the different parts, at others in the direction of greater simplicity or complication. The mucous glands may be absent, and the flagellum greatly reduced in size, or absent altogether.

The Dart Sac.—A remarkable part of the reproductive system in many of the true Helicidae is the so-called dart, Liebespfeil, or telum veneris. It consists of ‘a straight, or curved, sometimes slightly twisted tubular shaft of carbonate of lime, tapering to a fine point above, and enlarging gradually, more often somewhat abruptly, to the base.’ The sides of the shaft are sometimes furnished with two or more blades; these are apparently not for cutting purposes, but simply to brace the stem. The dart is contained in a dart sac, which is attached as a sort of pocket to the vagina, at no great distance from its orifice. There are four different forms of sac. It may be single or double, and each of these divisions may be bilobed, each lobe containing one dart at a time. In Helix aspersa the dart is about ⁵/₁₆ in. in length, and ⅛ in. in breadth at its base (see Fig. 54).

It appears most probable that the dart is employed as an adjunct to the sexual act. Besides the fact of the position of the dart sac anatomically, we find that the darts are extruded and become embedded in the flesh just before or during the act of copulation. It may be regarded, then, as an organ whose punctures induce excitement preparatory to sexual union. It only occurs in well-grown specimens. When once it begins to form, it grows very rapidly, perhaps not more than a week being required for its entire formation.