380 MOLLUSCA. 



and Limnaea, which had been kept isolated all their lives, have 

 been known to lay eggs, from which young were hatched ; but these 

 may have been cases of parthenogenesis, a phenomenon which has 

 of late been observed in quite unexpected quarters. 



In some Gastropods (Paludina, Ampullaria, and some species of 

 Murex) two kinds of spermatozoa are produced in the same gonad. 

 One of these is hair-like and the other vermiform. The meaning 

 of the phenomenon is unknown, but it appears that the former 

 alone conjugate with the ovum. 



Fertilization is nearly always internal, and brought about by copulation, which 

 in hermaphrodite forms with contiguous openings for male and female may be 

 reciprocal ; but in Chitonidae, Patella, Haliotis, and other forms without a penis, 

 it is probable that it takes place externally in the sea (it certainly does so in 

 Chiton). In such forms also the eggs are often laid singly and without any 

 accessory protective capsule (Patella, Haliotis, Chiton), though in Fissurella they 

 are embedded in jelly. In aquatic forms the eggs are generally united in masses 

 of gelatinous matter, which may be rounded, elongated, or ribbon -shaped, and 

 are attached to plants or other bodies (fresh-water Pulmonates, Opisthobranchs, 

 Bithynia, Valvata, Heteropoda, etc.). In Streptoneura several eggs floating in 

 albumen are generally enclosed in hard and coriaceous capsules or cocoons, and 

 it often happens that all the eggs in a cocoon do not develop, some serving as 

 food for the others. The eggs of Natica are embedded in albumen mixed with 

 sand, and have the appearance of a sandy ribbon. In some Streptoneura the 

 eggs are attached to a part of the body or shell, e.g. Vermetus on the inside 

 of the shell, Nerita on the outside. In lanthina they are attached to the float. 



In Stylommatophora the eggs are laid singly, and are often remarkable for 

 their size ; in Bulimus, Aehatina, and some species of Helix they are as large as 

 the eggs of small birds, and provided with calcareous shells. In such cases, 

 however, the ovum itself is small, the mass of the egg consisting of albumen. 

 The young of Helix Waltoni when first hatched is about the size of a full-grown 

 Helix hortensis. In Helix hortensis and aspersa the eggs are laid in hollows in 

 the ground and covered with earth. In some species of Paludina, Melania, 

 Littorina, Cymba, lanthina, Clausilia, Helix, Pupa, and Vitrina the eggs 

 develop in the uterus, and the young are born fully developed. 



The development begins with an unequal segmentation leading to 

 the formation of a blastula and gastrula. The blastopore varies in 

 its fate : it often has a slit-like form, and as a rule it closes up from 

 behind forwards either entirely or persists as the mouth-opening. 

 In Paludina, however, it persists as the anus. There is a 

 well -developed stomodaeum, giving rise to the buccal cavity and 

 a short proctodaeum. The mesoderm is derived from the endoderm 

 near the blastopore (or according to Tonniges from the ectoderm 

 along the site of the blastopore in Paludina), and often gives 

 rise to two mesoblastic bands, which in Chiton and some other 

 forms early exhibit a cavity. In Chiton, however, the fate of 



