EVOLUTION 105 
The process continuing as specialization proceeds, 
the shell ever tends to decrease in size till it remains 
solely as a protector for the more vital organs as in 
the Tectibranchs (Plate XII., Figs. 14-19), or dis- 
appears as in the rhipidoglossate Titiscania and the 
Nudibranchs (Plate XII., Figs. 20-28). 
A similar reduction and disappearance takes place 
among the pelagic forms. Light as Ianthina shells 
are, they are substantial compared to the glassy films 
carried by the Heteropods (Plate XXVI., Figs. 17-22) 
and shell-bearing Pteropods (Plate XXVI., Figs. 1-9), 
while Phyllivrhoé and the shell-less Pteropods (Plate 
XXVI., Figs. 10-16) have discarded all covering 
whatsoever. 
The freshwater Gastropoda, save those few that 
inhabit turbulent waters, have, as might be expected, 
thin shells; but though Amphipeplea (Plate XIIL., 
Fig. 5) and Physa (Plate XIII., Fig. 7) tend to over- 
flow their shells, an absolutely shell-less example 
remains to be discovered. 
On land heavy shells are certainly at a discount, 
and though some such occur among the Auriculidz, 
in certain species of Strophochilus, in Leucochroa (where 
it serves as a protection against extreme heat), and 
many of the Cyclophoride ; still, viewed broadly, the 
tendency, as might be expected, is to a lightening and 
diminution of the shell to the point of disappearance, 
and this more especially in the carnivorous and semi- 
carnivorous forms. In fact, nearly all the families of 
