Royal Physical Society. 193 



expands its many-coloured branchial plumes ; Lepralia and 

 other Polyzoa display their vase-like crowns of ciliated tenta- 

 cles ; and forests of Zoothamnium (an arborescent vorticella 

 resembling a miniature sea-fan) extend their branches and 

 wheel-bearing heads, or quickly coil up their writhing stalks 

 in a compact ball at the approach of danger. All these, and 

 many other forms of life, hurried through the water on the 

 house of the scrambling crab, doubtless profit by the constant 

 renewal of the vital element, and partake of the good fare 

 enjoyed by the eremite within. 



2. One of the most frequent cotenants of the abode of the 

 Pagurus is the Hydr actinia echinata. Shells infested by this 

 zoophyte appear to be covered by a white shaggy fleece, 

 which consists of a host of hydroid polyps, closely aggre- 

 gated together as the stalks in a field of corn. 



These polyps appear to be careless of the rough motions of the 

 crab, and wave loosely to and fro as it jolts along over the rocks ; 

 but when any part of the colony is rudely touched, the whole of 

 the polyps contract en masse, and seem to sink down into the 

 substance of the shell. A close examination shows that this con^ 

 sentaneous action on the part of the polyps is caused by their 

 being all developed from a common membranous basis, which 

 spreads itself in a continuous thin layer over the shell, and 

 being intimately connected with their tissues, forms a bond of 

 sympathy between the whole assemblage. 



3. The subject of the present communication is by no means 

 new to science. It has been treated of by Fleming, Johnston, 

 Gray, Couch, Hassall, M'Gillivray, Van Beneden, and espe- 

 cially by Quatrefages, whose monograph on this zoophyte 

 is replete with interest. After reading the memoirs of these 

 philosophers, I became convinced that a complete conception 

 of the morphology of Hydractinia had not been attained by any 

 of them. This I had previously been attempting to work out 

 by the examination of some hundreds of specimens, many of 

 which had existed for months confined in glass vessels, and 

 propagating themselves by stolons and sexual reproduction. 

 The conclusions I have come to may be briefly stated as fol- 

 lows : — In Hydractinia, the polypary, or common connecting 

 membrane is to be considered as an individual animal, possess- 



