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]