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On the Urticating Threads of Actinia parasitica. 



By Francis Alfred Bedwell, M.A., F.R.M.S. 



{Read January 24th, 1879.) 



Plate X. 



The line of the Dorsetshire coast near Swanage, magnificently 

 iron-bound though it is to the westward, nevertheless affords 

 from Durlstone Head eastward and northward, to Studland Bay, 

 a fair hunting ground for seaside study. When I first saw an 

 urticating thread it was at Durlstone Head. I was seeking micro- 

 scopical objects there, when a small pool attracted my attention ; it 

 displayed a score or more of white filaments, which radiated 

 to every point of the compass, from some central object that lay 

 concealed at the bottom of the pool. I had no sooner seen them 

 than the first returning wave of the flood tide filled the pool and 

 obliterated the sight ; it puzzled me exceedingly at the time, as I 

 was quite ignorant of its meaning, but, as I found out soon afterwards, 

 the filaments were the urticating threads (Acontia, Gosse) of Actinia 

 parasitica. This actinia, as is well known, attaches itself to the 

 whelk shell, which the hermit crab inhabits, and it is carried by the 

 crab into the lobster pots, which are laid in deep water along that 

 coast. It is not remarkable for that outward splendour which makes 

 some of these creatures such delightful studies externally ; but 

 it has some excellent qualities recommending it to the student ; 

 one is that it rarely keeps its beautiful disc closed for long, 

 while it carries about 50 of these threads ; and they are 

 organs of extraordinary interest. A thread varies from ten to 

 four inches in length — fine as very fine sewing thread, and yet 

 strong enough singly to bear the weight of actinia, crab and 

 shell, and draw them up to the surface of the water without break- 

 ing ; the threads are greasy to the touch, and ooze through the fingers, 

 however tightly held. If a piece is cut off and examined under the 

 microscope, it is seen to be literally a crowded mass of elliptical cap- 

 sules, all set with the long axis radiating to a central longitudinal 

 axis, so as to give (as I suppose) an imaginary transverse section 



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