392 PHYLUM MOLLUSCA. 
It seems to serve as a lubricant which facilitates the 
expulsion of a calcareous dart and the copulation. 
(g) Finally, between the entrance of oviduct and penis 
into the terminal aperture there lies a firm cylindrical 
structure, larger than the penis and with muscular walls. It 
is the Cupid’s Dart Sac, and contains a pointed calcareous 
arrow (spiculum amoris), which is jerked out previous to 
copulation. The dart is sometimes found adhering to the 
foot of a snail, and after copulation the sack is empty, soon, 
however, to be refilled. 
When two snails pair, the genital apertures are dilated, the 
protruded penis of one 
is inserted into the 
aperture of the other, 
and the spermatophore 
of each snail is trans- 
ferred to the recepta- 
culum of the other. 
The large eggs are laid 
in the earth in June 
and July. Each is sur- 
Fic. 212.—Diagram of larva of Pala. rounded by gelatinous 
dina.—After Erlanger. material acquired in the 
Ec., Ectoderm; £x., endoderm 5 v, velum, oviduct and by an elastic 
with cilia; g., gut-cavity; S.c., segmenta- 
tion cavity; c.Z., coelom pocket from gut; but calcareous shell. 
bi.g., blastopore groove closed, except at Segmentation is total 
6l., which becomes the anus. The origin » 
of ‘the mesoderm from a gut-pocket has as but slightly unequal. AS 
yebooly Been ilespabed 1 in Paludina among the snail is a terrestrial 
Gasleropod, there is no 
trochosphere larva, nor more than a slight hint of the char- 
acteristic Molluscan velum. A miniature adult is hatched 
in about three weeks. The study of development may be 
more profitably followed in the pond-snail Zimncaus, where 
gastrula, trochosphere, and veliger can be readily seen. 
Second Type of Mottusca. The Fresh-water Mussel 
(Anodonta cygnea), one of the Lamellibranchiata 
Habit.—The fresh-water mussel lives in rivers and ponds. 
It lies with its head end buried in the mud, or moves 
slowly along by means of its ploughshare-like foot. Its food 
