Spermatozoa travel from the ovotestis (1) through the common duct (2 a.k.a. ovotestis duct or little hermaphrodite duct) to the albumen gland (3), thence through the prostate gland (4) and vas deferens (5) to the penis (6) where they are formed by its epiphallus (6e) and flagellum (6f) into a spermatophore (long thin package). The spermatophore is ejected through the everted penis into the other snail’s vagina (7). Fertilization is delayed for a few weeks, sometimes months, while a store of allosperm is accumulated in the diverticulum from couplings with other partners. To try to ensure that a single partner’s sperm does not monopolize the fertilization, each allosperm deposit is reduced by the bursa copulatrix (8) digesting some of it on its transit from the vagina along the bursal duct (9) towards the diverticulum (10 small on this species). The insertion of a love dart from its sac (11) by the sperm-inserting partner earlier in the mating process partially counters the reduction of its sperm contribution as the dart is coated from the mucus glands (12) with mucus containing chemicals that cause contraction waves in the vagina (7) which intermittently constrict the duct branch (13) to the bursa copulatrix (8), and increase the contractions of the diverticulum (10) which probably ease the passage of the spermatophore into it. Allosperm are stored in the diverticulum until required.
Ova pass from the ovotestis (1) through the common duct (2 a.k.a. ovotestis duct) to the albumen gland (3) where they each receive a coating of nutrient albumen and move into the fertilization chamber (14). Stored allosperm are released to swim down the bursal duct (9) and up the oviduct (15) 37Ch 37 Cepaea hortensis. Dissection with reproductive organs isolated but not spread out. Pennine foothills, V.C. Derbyshire, England. September 2018. to fertilize the ova in the fertilization chamber. The fertilized ova then travel down the oviduct, through the atrium (16), and out of the genital pore (17) to be laid in a cavity excavated in the soil. Hatching occurs after some weeks; the period depending on temperature and humidity. Crawling miniature adults emerge as there is no larval stage.
Other Hypotheses
The text above is based on Kerney & Cameron (1979) and Cameron (2008) except the part in italics which is based on Lodi & Koene (2016). The functions of love darts, the bursa copulatrix and diverticulum are not fully understood.
Kerney & Cameron (1979), and most other authors pre 2016, were of the opinion that a received spermatophore is broken down in the diverticulum and the released allosperm stored in the bursa copulatrix.
Cordero (accessed October 2018) listed the following hypotheses about the love dart:
(1) it is a calcium gift used to entice the partner to act as female;
(2) it is a calcium gift used to entice the partner to act as male;
(1&2 mentioned and discounted by Zając & Kramarz, 2017).
(3) it is an honest signal of mate quality;
(4) it facilitates species recognition;
(5) it is a means of transporting chemicals to a potential mate;
(6) it is used to prevent unwanted matings.

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