4 46 Natural History of British Zoophytes. 



Additional Note. 

 The only extensive series of experiments we have on the composition of 

 Polypidoms are those of Mr Hatchett, published in the Philosophical Transac- 

 tions for 1800, and the subsequent progress of zoological and chemical science 

 requires a new investigation of the subject. The general results of Mr Hatch- 

 ett's experiments are, " that the Madrepores and Millepores are formed of a ge- 

 latinous or membranaceous substance, hardened by carbonate of lime, the differ- 

 ence consisting only in the mode in which these materials are combined : that 

 in the Tubipora, Flustra and Corrallina, some phosphate of lime is mixed with 

 the carbonate of lime : that in the Isis the basis is a regularly organized mem- 

 branaceous, cartilaginous and horny substance, hardened by carbonate of lime, 

 one species only (the Isis ochracea) yielding also a small proportion of phos- 

 phate of lime. That the hardening substance of the Gorgonia nobilis is like- 

 wise the carbonate of lime, with a small portion of phosphate ; but that the 

 matter forming the membranaceous basis consists of two parts, the interior being 

 gelatinous, and the external a complete membrane, so formed as to cover the stem 

 in the manner of a sheath or tube. That the other Gorgonia? consist of a horny 

 stem coated by a membrane, which is hardened by carbonate of lime. That the 

 sponges are of a nature similar to the horny stems of the Gorgoniee, and only 

 differ from these and from each other by the quality of texture. And lastly, 

 that the Alcyonia are likewise composed of a soft flexible membranaceous sub- 

 stance, very similar to the cortical part of some of the Gorgoniae ; and in like 

 manner slightly hardened by carbonate., mixed with a small portion of phosphate 

 of lime. 



" From this mass of evidence we collect, in general, that the varieties of bone, 

 shell, coral, and the numerous tribe of Zoophytes with which the last are con- 

 nected, only differ in composition by the nature and quantity of the hardening 

 or ossifying prirfciple, and by the state of the substance with which this princi- 

 ple is mixed or connected ; the gluten, or jelly, which cements the particles of 

 carbonate or phosphate of lime, and the membrane, cartilage, or horny substance 

 which serves as a basis, appearing to be only modifications of the same sub- 

 stance, which progressively graduates from a viscid liquid, or gluten, into a ge- 

 latinous substance, which again, by increased inspissation, and by the more or 

 less perfect degrees of organic arrangement, forms the varieties of membrane, 

 cartilage, and horn, which, it seems, form the peculiar differences of the several 

 species." Abst. Phil. Trans, i. 25-26. There is a very full analysis of Hatch- 

 ett's paper in Thomson's Syst. of Chem. v. p. 559, et seq. The paper itself 

 will be found in Phil. Trans, abridg. xviii. p. 706. 



IV. The British Zoophytes, divided into pour Orders. 



From what has been said in the preceding chapters, it seems to 

 follow that the Polypidom, considered by itself, will prove a fallacious 

 guide in any attempt to classify Zoophytes, if it is wished that that 

 classification (and other classification we hold next to valueless) 

 should give a correct representation of the structure of the animated 

 tenants, and distribute these into groups characterized by a unifor- 

 mity in the|leading traits of their anatomy. It is true, that La- 



