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 (Tsenioglossa) Plate XXXI I., Figs. 11 and 

 12), and Strombus Mauritianus (Taenioglossa), 

 simulating Conus Janus (Toxoglossa), while there 

 are equally noteworthy examples among the 

 Pelecypoda. Covalliophaga, a genus of Cyprinidae 

 (Eulamellibranchia), in its external appearance 

 closely resembles Lithodomus, one of the Mytilidae 

 (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 

 Veneridse, recently introduced into this country, 

 seemingly with Oysters from America, that at first 



