TRANSACTIONS OF SECTION D. 681 



for the species. On the other band, Clubb found that of the specimens of another 

 common sea-anemone, Actinia equijia, only 4L''l per cent, showed variations from 

 the normal mesenterial arrangement for the species. We have then, in these 

 examples, a set of organs which are very variable in one genus (Metridium), much 

 less variable in another (Actinia), aud perfectly fixed or rigid in another series of 

 genera (the Alcyonaria). 



Passing on, now, to the character ' shape.' Not many years ago the systematic 

 zoologists, who directed their attention to the sedentary Ccelenterates, based 

 their specific diagnoses very largely on the shape of tiie colonies. Thus we 

 have introduced sucli names as Millejwra alciconds, M. ramosa, M. plicata, 

 Madrepora cervicorvis, M. prolifera, M. imlmatn, Alcyonimn cligitatinn, A. 

 palmatmn, &c. &c. Zoologists are now agreed, however, that the shape of these 

 colonies is so variable that in most genera it is of very little value for the separa- 

 tion of species. In fact, I have elsewhere given reasons for holding the view 

 that the widely distributed and very variable genus Millepora is represented by 

 only one true species. But what is true for most sedentary Ccelenterates is not 

 true for all colonial Ccelenterates. In most of the genera and species of Pen- 

 natulida, for instance^ the shape of any one individual of a species is almost 

 identical with that of any other. A Tuniculina quudramjulavis, from the west 

 coast of Scotland, is similar in shape to one of the same species from the coast of 

 Norway. A Fennatula vwnai/i, from the reefs of Funafuti, is similar in shape 

 to one from Ceram. In other words, the character 'shape ' is extremely plastic in 

 Millepora and Madrepora, but very slightly plastic or almost rigid in Pennatula 

 and Funiculina. 



This difference in the plasticity of the character ' shape ' in Millepora and the 

 Pennatulids must be associated with the fact that the young Millepora colony is 

 unable to move from the spot where the larva settles, whereas the Pennatulid is 

 capable of moving from place to place throughout life. The Millepora colony 

 must either accommodate itself to the environment in which it begins life or 

 perish, hut the young Pennatulid can, within certain limits, travel to the environ- 

 ment that suits itself. 



The shape of a growing coral or sedentary Alcyonarian on a reef must accommo- 

 date itself to the depth of water, the position of neighbouring zoophytes to 

 itself, the direction of the tides, and other influences ; and such a power of 

 accommodation is essential for the species in the struggle for existence on the 

 coral reef. But in the case of the Pennatulid, the natural or normal shape is adapted 

 to a less variable series of environmental conditions, and it has sufficient power of 

 movement to shift itself into localities where the environment is suitable for it. 

 In other words, the power of movement is associated with a loss of plasticity of the 

 character ' shape.' 



But the growth of corals may be affected in other ways. A great many of 

 these forms of life harbour a small fauna of epizoic Crustacea, mollusca, and worms, 

 and the ramification or surface is often affected by these in a remarkable way. I 

 have elsewhere pointed out that the character of certain specimens of Millepora, 

 which is known as verrucose, is due to a modification of the growth round epizoic 

 barnacles. Semper has shown that the curious cage-like growths seen on the 

 branches of Seriatopora and Pocillopora are galls produced by the action of certain 

 species of crabs. In a recent paper I have also given reasons for believing that 

 the tubular character of the stem and some of the branches of the genus Soleno- 

 caulon is due to the action of certain Crustacea belonging to the family Alpheidje, 

 and that when these Alpheids are not present the form with a solid stem hitherto 

 known as the genus Leucoella is produced. 



But whilst some genera of corals and Alcyonaria are plastic in this way, others 

 are not. These coral galls may bo found on the Milleporas and Madreporas of 

 a certain portion of a reef and be absent from all the other genera of neighbouring 

 corals. The crab-galls that are found so commonly and in such abundance upon 

 Pocilloporas and Seriatoporas in certain parts of the Pacific and elsewhere are 

 found only in cases of extreme rarity in other corals. 



Many other cases could be given to show that in some genera the rcenenchvm 



