The dart is almost confined to Helicidae, a certain number of exceptions being known which border on Helix. Hyalinia nitida and excavata are the only British species, not Helices, which are known to possess it. It has not been noticed to occur in the slugs, except in the N. American genus Tebennophorus. About one-third of the British Helices are destitute of the dart.[260] H. rufescens possesses a double bilobed sac, but only two darts, which lie in the lower lobes. It does not use the darts, and could not do so, from the relative sizes of dart and sac; it has often been watched when uniting, but the use of the darts has never been observed. From this it has been inferred that the darts are degenerate weapons of defence, and that they were in fact at one time much stronger organs and more often used.[261] This theory, however, does not seem consistent with the whole circumstances of the occurrence, position, and present use of the darts.

Hermaphrodite Mollusca.—(b) Digonopora.—As an example of the Digonopora or hermaphrodite Mollusca with separate generative apertures for the male and female organs, we may take the common Limnaea stagnalis (Fig. 55). It will be seen from the figure that the relative positions of the hermaphrodite gland and duct, and of the albumen gland, are the same as in Helix. When the oviduct parts company from the vas deferens, it becomes furnished with several accessory glands, one of which (Gl.E.) probably serves as a reservoir for the ova, and answers more or less to a uterus. The tube leading to the spermatheca is short, and there is no divergent caecum. The female orifice lies near to the external opening of the branchial cavity. The vas deferens, which is very long, is furnished with a large prostate gland. The penis sac is greatly dilated, and there is no flagellum. The male orifice is behind the right tentacle, slightly in advance of the female orifice (compare Fig. 102).

Most of the Opisthobranchiata, but not all, have separate sexual orifices. Numerous variations from the type just described will be found to occur, particularly in the direction of the development of accessory glands, which are sometimes very large, and whose precise purpose has in many cases not been satisfactorily determined.

Pelecypoda.—In the dioecious Pelecypoda, which form the great majority, the reproductive system is simple, and closely parallel in both sexes. It consists of a pair of gonads, which are either ovaries or testes, and a pair of oviducts or sperm-ducts which lead to a genital aperture. The gonads are usually placed symmetrically at the sides or base of the visceral mass. The oviduct is short, and the genital aperture is usually within the branchial chamber, thus securing the fertilisation of the ova by the spermatozoa, which are carried into the branchial chamber with the water which passes through the afferent siphon.

Hermaphrodite Pelecypoda are rare, the sexes being usually separate. The following are assured instances: Pecten glaber, P. jacobaeus, P. maximus, Ostrea edulis, Cardium norvegicum, Pisidium pusillum, Cyclas cornea, Pandora rostrata, Aspergillum dichotomum, and perhaps Clavagella. The greater number of these have only a single genital gland (gonad) on each side, with a single efferent duct from each, but part of the gland is male and part female, e.g. in the Pectens above mentioned. Pandora and Aspergillum have two distinct glands, respectively male and female, on each side, each of the two glands possessing its separate duct, and the two ducts from each side eventually opening near one another. It appears probable that the Septibranchiata (Cuspidaria, Poromya, Lyonsiella, etc.) must also be added to the number of hermaphrodite Pelecypoda which have separate male and female glands.

It is worthy of remark that all the hermaphrodite Pelecypoda belong to forms decidedly specialised, while forms distinctly primitive, such as Nucula, Solenomya, Arca, and Trigonia are all dioecious. In Gasteropoda similarly, the least specialised forms (the Amphineura, with the exception of the Neomeniidae, and the Rhipidoglossa) are dioecious. It is possible therefore that in the ancestors of the Mollusca the separation of the sexes had already become the normal type of things, and that hermaphroditism in the group is, to a certain extent, a sign or accompaniment of specialisation.[262]

Development of Fresh-water Bivalves.—The vast majority of fresh-water bivalves either pass the larval stage entirely within the mother, and do not quit her except in a perfectly developed form (Cyclas, Pisidium), or assume a mode of development in which free larvae indeed occur, but are specially modified for adaptation to special circumstances (Unio). Cyclas and Pisidium, and no doubt all the kindred genera, preserve their ova in a sort of brood-pouch within the gills, in which the ova pass the earlier stages of their development. But, even so, the larva of these genera retains some traces of its original free-swimming habits, for a rudimentary velum, which is quite useless for its present form of development, has been detected in Cyclas.

The larva of Dreissensia (see Fig. 47, A), so far as is at present known, stands alone among fresh-water bivalves in being free-swimming, and to this property has been attributed, no doubt with perfect justice, the fact of the extraordinarily rapid spread of Dreissensia over the continent of Europe (chap. xvi.). In expelling the ova, the parent slightly opens the shells and then quickly closes them, shooting out a small point of white slime, which is in fact a little ball of eggs. The general course of development is precisely parallel to that of marine Pelecypoda, greatly resembling, so far as form is concerned, certain stages in the growth of the larvae of Modiolaria and Cardium, as figured by Lovén.[263]

In June and July the larvae appear in large numbers on the surface of the water, when in spite of their exceedingly small size, they can be captured with a fine hand-net. They pass about eight days on the surface, feeding apparently on minute floating algae. During this time, the principal change they undergo is in the formation of the foot, which first appears as a small prominence midway between the mouth and anus, and gradually increases in length and flexibility. When the larva sinks to the bottom, the velum soon disappears entirely, the foot becomes exceedingly long and narrow, while the shell is circular, strongly resembling a very young Cyclas.

Larvae of Unionidae.—The early stages of the development of Unio and Anodonta (so far as the species of North America, Europe, and Asia are concerned) is of extreme interest, from the remarkable fact that the young live for some time parasitically attached to certain species of fresh-water fishes. In order to secure this attachment, the larva, which is generally known as Glochidium, develops a long filament which perhaps renders it aware of the neighbourhood of a fish, and also a larval shell furnished with strong hooks by which it fastens itself to the body of its unconscious host (Fig. 56). According to some interesting observations made by Mr. O. H. Latter,[264] the ova pass into the external gill of the mother, in which is secreted a nutritive mucus on which they are sustained until they arrive at maturity and a suitable opportunity occurs for their ‘being born.’ If this opportunity is deferred, and the Glochidia mature, their so-called ‘byssus’ becomes developed, and by being entangled in the gill filaments of the parent, prevents their escaping. It is interesting to notice that, when the nutritive mucus of the parent is used up, it becomes, as it were, the turn of the children to provide for themselves a secondary mode of attachment.

The mother Anodonta does not always retain the Glochidium until fish are in her neighbourhood. Gentle stirring of the water caused them to emit Glochidium in large masses, if the movement was not so violent as to cause alarm. The long slimy masses of Glochidium were observed to be drawn back again within the shell of the mother, even after they had been ejected to a distance of 2 or 3 inches.

It is a mistake to assert that the young Glochidium can swim. When they finally quit the mother, they sink to the bottom, and there remain resting on their dorsal side, with the valves gaping upwards and the so-called byssus streaming up into the water above them. There they remain, until a convenient ‘host’ comes within reach, and if no ‘host’ comes within a certain time, they perish. They are evidently peculiarly sensitive to the presence of fish, but whether they perceive them by smell or some other sense is unknown. “The tail of a recently killed stickleback thrust into a watch-glass containing Glochidium throws them all into the wildest agitation for a few seconds; the valves are violently closed and again opened with astonishing rapidity for 15–25 seconds, and then the animals appear exhausted and lie placid with widely gaping shells—unless they chance to have closed upon any object in the water (e.g. another Glochidium), in which case the valves remain firmly closed.”

In about four weeks after the Glochidium has quitted its host, and the permanent shell has made its appearance within the two valves of the Glochidium, the projecting teeth of the latter press upon the ventral edge of the permanent shell, at a point about half way in its lengthward measurement, retarding the growth of the shell at that particular point, and indenting its otherwise uninterrupted curve with an irregular notch or dent. As growth proceeds, this dent becomes less and less perceptible on the ventral margin of the shell itself, but its effects may be detected, in well-preserved specimens, by the wavy turn in the lines of growth, especially near the umbones of the young shell.

Mr. Latter found that all species of fish with which he experimented had a strong dislike to Glochidium as an article of food. Sometimes a fish would taste it “just to try,” but invariably spit it out again in a very decided manner. The cause of unpleasantness seemed not to be the irritation produced in the mouth of the fish by the attempt of the Glochidium to attach itself, but was more probably due to what the fish considered a nasty taste or odour in the object of his attentions.

The following works will be found useful for further study of this portion of the subject:—