356 MOLLUSC A. 



are fertilised. The second structure associated with the 

 female duct is a conspicuous mucus gland, formed of two 

 sets of finger-like processes. The mucus secretion of this 

 gland is very abundant during copulation, and as it contains 

 not a little lime, it is possible that it may form the calcareous 

 shells of the eggs. 



(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 

 skin of a snail, and after copulation the sac is empty, soon, 

 however, to be refilled. 



When two snails pair, the genital apertures are dilated, 



*- * 



^Ec 



bl. 



big 



FIG. 1 1 6. Diagram of larva of Paludina. (After ERLANGER. ) 



c., ectoderm; En., endoderm; v. velum, with cilia; g.> gut 

 cavity; S.c., segmentation cavity; c.p. , coelome pocket from gut; 

 bl.g., blastopore groove closed, except at bl., which becomes the 

 anus. 



the protruded penis of one is inserted into the aperture of 

 the other, and the transference of a spermatophore is thus 

 effected. 



The eggs are laid in the earth in June and July. Each is 

 surrounded by gelatinous material acquired in the oviduct, 

 . and by an elastic but calcareous shell. 



Segmentation is total but slightly unequal. As the snail 

 is a terrestrial Gasteropod, there is no trochosphere larva nor 

 more than a slight hint of the characteristic Molluscan 

 velum. A miniature adult is hatched in about three weeks. 



