Life of the Snails 25 



sexuality. Dual sexuality or hermaphroditism as found in the pulmonates is 

 also known in some species of Acmaea Limpets, Janthina, Odosto7nia, Stilifer, 

 Valvata and the Paper Moon Snail, Velutina. The sexuality of this type, 

 however, is more of the consecutive type, in which the gonads at first pro- 

 duce sperm and later in the season only eggs. 



Sex reversal is especially characteristic of the Slipper Shell family. The 

 best known examples belong to the Cup-and-saucer Shells, Calyptraea and 

 Crucibulum, and the true Slipper Shells, Crepidula. Individuals function as 



Figure 7. Sex reversal in the Slipper Shells, Crepidula. a to e, animal with shell re- 

 moved to show the development of the verge in the male phase; f and g, atrophy 

 of the verge and the change to the female phase; h, a group of attached Crepidula 

 forjiicata, showing the smaller males ( (J ) at the top and the females ( $ ) below; 

 i, Crucibidum spinosum with the small male attached to the female. (After W. R. 



Coe 1943.) 



the male sex when young and as females when fully grown. The change-over 

 may be gradual with the individual being ambisexual for a short period, or 

 the male phase may suddenly disappear with the loss of its associated organs, 

 and the female organs may then quickly develop. The males are much 

 smaller than the females. In most species, each young male tends to creep 

 about until it finds an individual of the same species in the female phase, 

 whereupon it attaches itself to the dorsal side of the female's shell in a posi- 

 tion adjacent to the female copulatory organs (fig. yi). In other species the 



