94 THE LIFE OF THE MOLLUSCA 



gillivvaya (Plate XXVI. , Fig. 29) is probably the 

 young of Dolium, and under the name Sinusigera 

 (Plate XXVI., Fig. 28) a large number of the young 

 of very different genera has been included. 



To the curious parasitic (glochidial) stage of the 

 young freshwater Mussels and Philobrya that succeeds 

 the veliger stage allusion has already been made 

 (ante, p. 82). 



There is usually a marked difference between the 

 embryonic shell (" protoconch " of Gastropods, " pro- 

 dissoconch " of Pelecypods) and the adult, the junc- 

 tion between the two being very marked, and indicat- 

 ing a pause in the growth of the shell, during which 

 time the young animal was completing its growth in 

 other respects (Plate IV., Fig. 2 ; Plate XXIX., Figs. 

 14 and 15). 



The Gastropod shell in the very early stages of its 

 formation is exogastric — i.e., coiled forward over the 

 head of the animal (Plate IV., Fig. 1, C)— but it 

 speedily takes on the right or left spiral of the adult. 

 Even in the Limpet (Patella) and Keyhole Limpets 

 (Fissurellidse) in their embryo stages have coiled 

 exogastric shells. The Fissurellidae are further 

 interesting in that the characteristic perforation, 

 beneath which the posterior termination of the ali- 

 mentary canal lies, first appears as a slit in the 

 margin of the young shell. In Emarginula (Plate 

 VII., Fig. 1) the slit persists through life, the hinder 

 end being constantly filled up as the animal grows ; 



