360 EEPOET— 1892. 



Of course a list of specimens collected during a montli's stay at 

 Plymouth cannot be taken as by any means a complete list of forms 

 occurring there, and, while expressing my thanks to tbe British Associa- 

 tion for so kindly allowing me the use of its table last year, I should 

 like at the same time to express the hope that on some future occasion 

 it will enable me to continue the work, and, having got over the drudgery 

 of mere labelling, to pass on to some of the many interesting morpho- 

 logical problems connected with Polychsets which I'emain to be worked 

 out. 



II. Heport on the Occupation of the Table. Some preliminary notes on the 

 anatomy and habits of Alcyonium digitatum. By Mr. Sydney J. Hick- 

 son, M.A., Fellow of Downing College, Cambridge. 



Alcyonium digitatum is one of the most difficult Ccelenterates to kill 

 in a fully expanded condition. In the first place it is only extremely 

 rarely that the large proportion of the polypes of a specimen in an 

 aquarium fully expand themselves, and when they are in that condition 

 the slightest touch or irritation of any part of the colony causes an 

 immediate contraction of the tentacles. Again, when a favourable 

 opportunity arises it is found that all the neutral killing reagents, such 

 as corrosive sublimate, &c., fail to kill the polypes before they have time 

 to partially retract. The only method that gives tolerably satisfactory 

 results is Lo Bianco's No. II. chromo-acetic acid method, and this of 

 course partially dissolves the calcareous spicules. 



When a living specimen of an Alcyonium digitatum is examined in 

 an aquarium the polypes may frequently be observed in various stages of 

 expansion and retraction. Sometimes all the polypes are completely 

 retracted, but I have never yet observed in any specimen all the polypes 

 fully expanded at the same time. By far the most frequent condition of 

 the Alcyonium is one in which a few polypes here and there are fully 

 expanded; others expanded but with their tentacles contracted, and 

 others only just protruding from the surface of the colony. 



These two stages are the normal ones that each polype passes through 

 in reaching complete reti'action from complete expansion. When the 

 polype is completely expanded both the body-wall and the tentacles are 

 delicate and transparent. 



The first stage in the retraction is the contraction of the tentacles. 

 The crown of the polype becomes roughly octagonal in shape with an 

 obtuse solid knob, the contracted tentacle, at each angle. 



In the next stage the contracted tentacles bend over towards the 

 mouth, and concurrently with the retraction of the body of the polype 

 they sink into a circular fold of the body- wall. 



The invagination of the polype then proceeds at the base until the 

 crown covered by the fold of body-wall sinks below the surface of the 

 •colony. 



When the crown has sunk below the surface of the colony the aperture 

 is closed by the folding of the delicate body-wall of the base of the 

 polype over the crown ; but when the colony enters into a state of com- 

 plete contraction, as it does, for example, when it is taken out of the 

 water for a few minutes, the tough obtuse surface of the colony contracts 

 over this delicate base, leaving only a star-like slit to mark the position 

 of the retracted polype. 



