34 PROCEEDINGS OF THE 



That such cause and effect are not without parallel in the 

 invertebrate world we have not far to seek. For instance, look 

 at the thickness attained by the shells of some oysters, arising from 

 the rapid deposition of calcareous matter by the mantle of the 

 mollusc to counteract the ravages of Cliona, and other similar 

 parasites. Again, observe the commercial use of this peculiarity in 

 some mollusca made by that ingenious and clever people, the 

 Chinese, who introduce small images and reliefs below the mantle 

 of a species of Anodon, and thus produce most peculiar structures 

 quite foreign to the nature of the shell, simply by the irritation 

 caused by their projection against the inner surface of the mantle. 



Again, if any further example of the formation of additional 

 matter consequent on the action of extraneous bodies is required, 

 there may be mentioned that of certain Annelides on the corallum 

 of the Stylasteridae. According to Mr. Moseley they " are liable 

 to become much distorted in growth by the presence upon them of 

 parasites of various kinds, each of which appears by the special 

 kind of irritation which it offers to produce a particular form of 

 abnormal growth in the part of the corallum it infests, producing 

 thus, as it were, an animal gall. The commonest distortion is the 

 reduction of the stem of a coral or branch, or of one side of these, 

 into a hollow canal or deep furrow, more or less roofed over by a 

 thin wall. This condition is produced by the adherence to the 

 growing stem of an Aphroditacean Annelid. . . . On Errina 

 labiata a parasitic filiform Nemertean also occurs, which twines 

 itself round the tips of the branches in many coils. The branches 

 thus irritated grow out into a burr -like mass of projecting points, 

 which are evidently hypertrophied dactylopore prominences. . . .* 



In conclusion, I have to express my thanks to Professor Geikie, 

 F.R.S., for the loan of specimens in the Collection of the Geological 

 Survey of Scotland, and to my colleague, Dr. H. Woodward, for 

 pointing out to me many curious specimens in the British Museum. 

 To Messrs. It. N. Peach and G. Sharman I am indebted for the 

 accompanying drawings. 



*The Croonian Lecture, "On the Structure of the Stylasteridae," 1878, 

 p. 469. 



