METHODS OF ATTACK AND DEFENCE 97 



green weeds. On the other hand, green specimens of 

 Hermcea dendritica are said to " avoid " red weeds, and to 

 " choose " the colour which matches their own. The 

 commonest species of Dorid in the Mersey, Lamellidoris 

 hilamellata, is richly coloured with yellow-brown and though 

 a striking object when placed in a white dish or in clear 

 water is quite inconspicuous on the dark purple-brown 

 rocks spotted with patches of adhering mud, sand, small algae, 

 and zoophytes. Again, Rostanga coccinea and Doris flammea 

 live on red sponges (the former on Microciona altrasangidnea) 

 to which their colour is closely assimilated ; Jorunna 

 johnstoni frequently occurs on stones associated with 

 Halichondria panicea which it resembles so closely in out- 

 line, colour, character of surface and projecting plumes as 

 to be hardly distinguishable ; Calma glaucoides eludes 

 observation among the fish-eggs which it eats, and many 

 forms bearing prominences or branched processes such as 

 ^gires, Doto, or Dendronotus have been noticed to correspond 

 exactly in colour with the Bryozoa or algae which form their 

 usual habitat. Finally, Hermcea bifida occurs on red seaweeds 

 of the genus Griffithsia. Though the shapes of plant and 

 animal do not correspond, yet in the water the outline of the 

 latter becomes blurred, and the only conspicuous features are 

 the red hepatic canals in the body, and their red diverticula 

 in the cerata. These are the exact counterpart of the fine 

 branches of the seaweed in form and colour, the latter, 

 indeed, being transferred from the plant to the animal 

 which sucks its juices. 



Examples of protective resemblance in shelled Gastero- 

 pods are less frequent, and a reason does not seem far to 

 seek. Walton (19 13) remarks, however, that the distribution 

 of Littorina ohtusata is coincident with that of Fuci, and that 

 the shells of the former often bear a striking resemblance 

 in both form and colour to the vesicles and conceptacles of 

 the algae on which they live, so much so that he has himself 

 mistaken the vesicles of the algae for the shell of the mollusc. 

 " The conceptacles are of shades varying from white to 

 bright yellow, and indeed the whole of the parts of the 



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