74 PROTECTIVE COLORATION— PARASITIC MOLLUSCA chap. 



even for tlie careful observer to distinguish the one from the 

 other. And, since fishes are known to be distinctly averse 

 to sponges of any kind as an article of food, this resem- 

 blance must be decidedly to the advantage of the Jorunna. 

 Another Nudibranch (Calma glaucoides A. and H.) imitates the 

 ova of certain fishes, on which it feeds. Its elongated and 

 depressed form of body, transparent integuments, and silvery 

 gray papillae combine to give it a strong resemblance to the 

 spawn of the fish, which is deposited on stones, the roots of 

 Laminaria^ etc.^ 



The common Lamellaria perspicua appears to possess the 

 power of protectively assimilating its colour, markings, etc., 

 to the Ascidians on which it lives. A recent case, occurring 

 off the Isle of Man, is thus described by Professor Herdman.^ 

 " The mollusc was on a colony of Leptoclinum maculatum, in 

 which it had eaten a large hole. It lay in this cavity so as 

 to be flush with the general surface ; and its dorsal integument 

 was not only whitish with small darker marks which exactly 

 reproduced the appearance of the Leptoclinum surface with the 

 ascidiozooids scattered over it, but there were also two larger 

 elliptical clear marks which looked like the large common cloacal 

 apertures of the Ascidian colony. . . . Presumably the Lamel- 

 laria escapes the observation of its enemies through being mis- 

 taken for part of the Leptoclinum colony ; and the Leptoclinum^ 

 being crowded like a sponge with minute sharp-pointed spicules, 

 is, I suppose, avoided as inedible (if not actually noxious through 

 some peculiar smell or taste) by carnivorous animals which 

 might devour such things as the soft unprotected mollusc." 



Parasitic Mollusca 



Various grades of parasitism occur among the Mollusca, 

 from the true parasite, living and nourishing itself on the tissues 

 and secretions of its host, to simple cases of commensalism. 

 Some authors have divided these forms into en do- and ecto- 

 parasites, according as they live inside or outside of their host. 



1 Hecht, Compte.s licMchis, cxv. p. 746. 



2 Conchulogist^ ii. p. 130. 



