128 CURIOUS FORMS OF EGG-LAYING. chap. 



than a form of protection for the ova, and is in no sense homol- 

 ogous 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 Hydrohia 

 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.^ 



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 

 Planorhis. 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.^ 



Union of Limax. — With regard to the act of union itself, 



1 Ilerdman, Proc. Liverp. Biol. Soc. iii. p. 30. 

 ■^ Garrett, Journ. Ac. Nat. Sc. Phil. viii. (1880). 



