
CRAB-GALL ON MILLEPORA. 81 
Crab-Gall on Millepora. 
By Sypney J. Hickson, M.A., F.R.S.,* 
Beyer Professor of Zoology, Owens College, Manchester. 
THE occurrence of gall-like growths on certain genera of branched Zoanth- 
arian corals, belonging to the genera Sideropora, Seriatopora, and Pocillopora, 
has been known to zoologists for many years. The best description of them 
may be found in Semper’s interesting book, “The Natural Conditions of 
Existence as they affect Animal Life.”’t 
The Crab that causes the growth of the gall is called Hupalocarcinus 
marsupialis. These galls are by no means rare. In nearly every Museum 
which possesses several specimens of these genera, examples of such galls are 
sure to be found; and in Seriatopora itself as many as nine or ten galls in 
different stages of formation are frequently seen in specimens less than a 
foot in diameter. 
The oceurrence of Crab-galls on JMillepora, however, has not, I believe, 
been hitherto recorded. It must, however, be of extremely rare occurrence, as 
it has never before come to my notice. During the past ten years I have 
examined the whole stock of Millepores in several Museums, and have 
received for examination from naturalists in various parts of the world the 
specimens of this genus that they have collected. 1 have noted the various 
parasites and commensals that are found on them, and the many variations 
and distortions of growth that they exhibit, and, therefore, have good reason 
for saying that the occurrence is a rare one. 
On the specimen in the Derby Museum, Liverpool (Fig., page 80), there 
are three galls, two complete and one in process of formation. 
Of the three galls the lowermost («) in the figure is the oldest. It is 
about 30 mm. long by 25 mm. broad, and the aperture is oval, 8 mm. by 
6 mm. in size. 
The cavity is large, its diameter being considerably greater than the 
greatest diameter of the aperture. The dried crab remains in this gall, but 
as it would be impossible to extract it without injury, 1 am unable to say 
more about it than that it is very much smaller than the cavity in which it 
lives. 
The second gall ()) is smaller and much more spherical in shape, its 
diameters being approximately 20 mm. The aperture is relatively larger 
than that of the other, being 6 mm. by 7 mm. in size. It contains no crab. 
The imperfect gall at the top (c) is widely open, and is formed of a net- 
work of Millepore branches imperfectly woven together. 
The extent of the malformation produced by these crabs need not be 
described as it is adequately represented in the illustration, but, I may add, 
the surface of the coral forming the outer wall of the gall shows no signs of 
unhealthiness or weakness. The cycles of pores are as numerous and as 
regular as in other parts of the corallum with normal growth, and the pores 
themselves are just as large and as well defined there as elsewhere. 
These galls cannot, therefore, be regarded as a disease, although they 
effect a considerable alteration in the normal growth of the corallum. 
Note.—The description of the gall-forming crab, given by Dr. Semper, 
and referred to above, is as follows :—‘‘So long ago as the year 1837 Stimp- 
son described a small crab, under the name of Hapalocarcinus marsupials, 

* The Director has to thank Prof. Hickson for contributing the above note on this 
specimen (31, xii., 96, 7 in the Derby Museum), belonging to a group of animals on 
which he has long been recognised as an authority. 
+ Kegan Paul, International Scientific Series, 1881. 
