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 XIL, Figs. 14-19), or dis- 

 appears as in the rhipidoglossate Titiscania and the 

 Nudibranchs (Plate XIL, 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 XXVL, Figs. 17-22) 

 and shell-bearing Pteropods (Plate XXVL, Figs. 1-9), 

 while Phyllirrhoe and the shell-less Pteropods (Plate 

 XXVL, 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 XIIL, 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 Auriculidae, 

 in certain species of Strophochilus, in Leucochroa (where 

 it serves as a protection against extreme heat), and 

 many of the Cyclophoridae ; 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 



