134 THE LIFE OF THE MOLLUSCA 
shade, and so warn their possessors of the approach 
of possible danger. 
With all this capacity for ready response to the 
action of environment, it is little wonder that 
members of widely different families and even orders, 
subjected to similar conditions of existence, should 
come to bear striking resemblance to one another. 
To the recurrence of the Limpet-shaped shell and 
the repetition of the Slug-like form, allusion has 
already been made (ante, pp. 103 and 106). Many 
other similar instances occur among Gastropods, 
notably Sycotypus (Rhipidoglossa), which resembles 
Ficus (Tzenioglossa) Plate XXXII., Figs. 1m and 
12), and Strombus Mauritianus (Teenioglossa), 
simulating Conus Janus (Toxoglossa), while there 
are equally noteworthy examples among the 
Pelecypoda. Coralliophaga, a genus of Cyprinide 
(Eulamellibranchia),. in its external appearance 
closely resembles Lithodomus, one of the Mytilide 
(Filibranchia) (Plate XXXII., Figs. 17 and 18), whose 
burrows it occupies (ante, p. 54). Dreissensia, the 
freshwater Zebra Mussel, belonging to the Sub- 
mytilacea (Eulamellibranchia), is constantly mis- 
placed on account of its striking similarity to Septifer 
with the Mytilacea (Filibranchia) (Plate XXXII., 
Figs. 13 and 14). Another noteworthy instance is 
that afforded by Petricola pholadiformis, one of the- 
Veneride, recently introduced into this country, 
seemingly with Oysters from America, that at first 
