94 THE LIFE OF THE MOLLUSCA 
gilliovaya (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 
(Fissurellide) in their embryo stages have coiled 
exogastric shells. The Fissurellide 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 
VIL., Fig. 1) the slit persists through life, the hinder 
end being constantly filled up as the animal grows; 
