440 Natural History of British Zoophytes. 



V. — The Natural History of British Zoophytes. By George John- 

 ston, M. D. Fellow of the Royal College of Surgeons of Edin- 

 burgh. (Continued from p. 247-) 



III. — -On the Structure and Formation of the 



POLYPIDOMS. 



The existence of a polypidom is not, as has been already men- 

 tioned, essential to a polype ; nor does it exercise, when present, 

 that great influence over the organization of its architects and ten- 

 ants which might have been anticipated. Thus the animal of the 

 madreporous Caryophyllsea does not essentially differ from the na- 

 ked Actinia ; and the gelatinous Hydra is a true representative of 

 the tenant of the sheathed Sertulariadse and Tubularia. No Asci- 

 dian polype, however, is ever found detached, and without a poly- 

 pidom ; and it is the same with all our native Astroida, but, per- 

 haps, the clustered animal-flower (Actinia sociata, Ellis, Zoanthus, 

 Cuv.) of the Carribaean sea, might take its place in this tribe with 

 greater propriety than in any other. 



In reference to their composition, Polypidoms may be divided into 

 1. the stony or calcareous, 2. the membrano-calcareous, and 3. the 

 horny and flexible ; but the line which separates these divisions is 

 often as uncertain and debateable as that which is traced between 

 the sister kingdoms. All are composed of the same materials, viz. 

 lime, and a gelatinous or membranaceous substance; and their pe- 

 culiar characters depend on the different proportions in which the 

 materials are mixed. The calcareous, which are hard and inflexible, 

 and, when dry, assume a white colour, consist principally of carbo- 

 nate of lime, with a small quantity of the phosphate of the same 

 earth, and the gelatinous matter which cements them into one co- 

 herent mass, is in sparing proportion : that proportion is so greatly 

 increased in the polypidoms of the second section, that when the 

 earthy ingredients have been removed by the action of diluted acids, 

 the structure retains its original form, and is, in fact, reduced to the 

 condition of the polypidoms of the third section, which contain no 

 lime, or very little of it, but are formed of a condensed gelatinous 

 membrane, which resembles horn in every essential property.* 



These diversities in their chemical composition appear to be of 

 little value, either in a physiological or systematical point of view, 

 for in every order of polypiferous zoophytes, we find calcareous and 

 horny polypidoms. A curious species of Actinia secretes a horny 



* See additional Note, p. 446. 



