No. 1)28] OXC'HIDIUM AND ADAPTIVE COLOBATION 417 



wlietlier tlicy be ' * species ' ' or not is immaterial. A simi- 

 lar phenomenon seems to occur in other genera of this 

 family, for v. Wissel (1898) notes that Plate (cf. Plate, 

 1893, 1894) collected two differently colored forms of 

 OucliidicUa coqiiimhensis Plate; in this instance the color 

 diflx'Tonce l)etween the two types, found in nature under 

 tlio same stones, was not so pronounced as we find it in 

 0. floridcuium. In an OnchidieUa whicli is very abundant 

 at Bermuda one of us has observed a corresponding sort 

 of color difference. The variation in question cannot be 

 in any wny a j^oxual one, for reasons already given and 

 because the animals are hermaphroditic. 



II. Oiirhninnii thnidmmm lives between tide levels. It 

 is ncvrv sct'u when tlie intertidal shore-zone is covered by 

 the sea; during this period it inhabits deep crevices and 

 cavities within the eroded rock. A dozen or more Onchi- 

 dia live together in this way. Their *'nest" opens to the 

 exterior l)v means of a small opening, usually further ob- 

 structed by till' ui'owih of Modiolus, wliich is almost in- 

 vai'ialiK iiiiitc iiicon-^] licnDii--. all Inumii the densely packed 

 small l.la<-k- •'mussels"" within and around the entrance 

 itself ma\- foi'in a ver\- deiinite patcli standing out clearly 

 amid tli(' ()live-l>r(.wn alua- coverinu' the surface of the 

 rock. When the tide falls so far as to have left the level 



