74 PROTECTIVE COLORATION PARASITIC MOLLUSCA chap. 



difficult even for the 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 re- 

 semblance must be decidedly to the advantage of the Jorunna. 

 Another ISTudibranch {Gahiut glaucoidcs 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 Lamcllaria pcrspicua 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 Leptoclimim 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 Leptodinum 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 

 Lamellaria escapes the observation of its enemies through 

 being mistaken for part of the Leptodinum colony ; and the 

 Leptodinum, 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 MoUusca 



Various grades of parasitism occm' 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 endo- and ecto- 

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



^ Hecht, Comptes Rendus, cxv. p. 746. 

 2 Conclwlogist, ii. p. 130. 



