GARSTANG : STRUCTURE AND HABITS OF JORUNNA JOHNSTONI. 5 I 



process is a small group of opaque white bodies which appear to be 

 gland-cells. 



I am inclined to believe that these curiously-formed tubercles 

 discharge both a defensive [spicules] and a tactile [tentacular 

 processes] function. So many fishes and invertebrates search for 

 their prey by the sense of touch that it is a distinct advantage 

 to a sedentary gastropod to have special tactile organs by which it 

 may be warned when to adhere more firmly to its place of attachment. 

 Every one knows how firmly a limpet clings to a rock after being 

 touched or slightly disturbed. 



The tubercles are uniformly distributed all over the back of the 

 nudibranch, except on the dark-brown patches, where they are more 

 scanty. 



For several years I have paid attention to the habits of Jornnna 

 Johnstoni, both in its native haunts and in aquaria, in order to see 

 whether the peculiarities of the species are in any sense adaptive. 

 I believe that a decidedly affirmative answer can now be given to 

 the question. Jorunna Johnstoni is a very convincing instance of 

 protective mimicry among marine animals — as complete in its own 

 way as the best of cases in the insect world. 



In a paper* published three years ago, I drew attention to the 

 remarkably sponge-like appearance of an individual of this species, 

 the conical tube of branchial plumes simulating the protruding 

 osculum of a small Halichondria. So many additional details in 

 this resemblance have since been forced upon me that I can no 

 longer doubt the reality of the mimicry ; and in this fact the key is 

 found to almost every peculiarity of form, marking, and habit which 

 distinguishes the species from its allies. 



The nudibranch lives on the same stones as the small 

 Halichondrice (provided with only one or two oscula), which it 

 so closely resembles, and the sponges are far more plentiful than 

 the nudibranch. 



The irregularly ovate contour of Jonuma Johnstoni, when at rest, 

 approximates closely to the form of the sponge. The osculum of 

 the latter is strikingly mimicked by the tube of branchial plumes. 



The general colour of the two forms is the same, and the slight 

 variations in tint exhibited by the nudibranch are also found in the 

 sponge. The pale brownish spots on the back of Jorunna Johnstoni 

 are seen again in the sponge, where they seem to indicate the 

 positions of the inhalent pores or ostia. 



The yielding, velvety, but spiculose surface of the sponge is 

 similarly found in the nudibranch, whose back is completely covered 



"Jour. Mar. Biol. Ass. (N.S.), I., 1889, pp. 174 and 178. See also same journal, 1890, p. 445. 



