392 



PHYLUM MOLLUSC A. 



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- 



bl 



.g c.p. 



rounded 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 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 Limnaus^ where 

 gastrula, trochosphere, and veliger can be readily seen. 



FIG. 212. Diagram of larva of Palu- 

 dina. After Erlanger. 



EC., Ectoderm; En., endoderm; v., velum, 

 with cilia ; ., gut-cavity ; S.c., segmenta- 

 tion cavity ; c.p., ccelom pocket from gut ; 

 bl.g., blastopore groove closed, except at 

 bl., which becomes the anus. The origin 

 of the mesoderm from a gut-pocket has as 

 yet only been described in Paludina among 

 Mollusca. 



Second Type of MOLLUSCA. 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 



