306 Mr Hickson, Some preliminary notes on the [Feb. 22, 



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, etc. fail to kill 

 the polypes before they have time to partially retract. The only 

 method that gives tolerably satisfactory results is Lo. Biancho'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 retraction 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 complete 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. 



The ectoderm is in the uninjured specimens composed of 

 several layers of cells. Under unhealthy conditions the superficial 

 layers are apt to slough. 



The stomodaeum of each fully-developed polype opens into 

 a long coelenteron that passes down to the base of the colony. 

 In a longitudinal section through a branch of the colony the 



