VERMES. 



Corala, other zoophytes, and lithophytes, have hard parts, 

 which are fometimes horny, fometimes calcareous, and fome- 

 timcs fpongy ; but which grow by fimple juxtapofition, or 

 at lead hkc (hells by the addition of fucceffive ftrata. In 

 fome their growth takes place externally, and the fenfible 

 fubftance envelopes the old ftrata by new ones, with which 

 it again covers itfelf. Such is the cafe with the lithophyta 

 and ceratopliyta. In others, the parts which have once at- 

 tained their proper hardnefs, no longer increafe in thick- 

 iiefs ; but new fhoots or branches are formed at their extre- 

 mities. Such are all the jointed zoophytes. 



There are fome minute obfervations on the texture, 

 courfe of the fibres, &c. of fhells, and fimilar fubftances, in 

 a paper by Mr. Beudant, entitled " Memoirc fur la Strudture 

 des Parties folides des Mollufques, Radiaires, et Zoophytes." 

 See Annales du Mufcum, t. xvi. p. 66. 



Chemical Compo/ilien of Shells, isfc For our knowledge 



•of the chemical compofition of thefe fubftances, we are 

 indebted principally to the excellent papers of Mr. Hat- 

 chett in the Philofophical Tranfaftions for 1799 and 1800. 

 Shells, like bones, confift of calcareous falts united to a 

 ■foft animal matter ; but in the former the lime is united 

 chiefly to carbonic acid, whereas in the latter it is united 

 to phofphoric acid. The predominating ingredient in fhells 

 is carbonate, in bones, pliofphate of lime. This conftitutes 

 the charafteriftic difference in their compofition. 



Mr. Hatchett divides Ihells into two clafies. The firft 

 are ufually of a compaft texture, refemble porcelain, and 

 have an enamelled furfacc often finely variegated. The 

 fliells belonging to this clafs have been diftinguiftied by the 

 name of porcellaneous fhells ; they are exemplified in the 

 voluta, cyprasa, &c. Thofe of the fecond clafs are ufually 

 covered with a ftrong epidermis, below which lies the fhell 

 in layers, and compofed of the fubftance known by the 

 name of mother-of-pearl : thefe he calls mother-of-pearl 

 fhells. The frefh-water mufcle, the halyotis iris, and the 

 turbo olearius, are examples. In the firft clafs there is a 

 fmall, in the fecond a large proportion of animal matter. 



Porcellaneous fliells contain fo little animal matter, that 

 they emit no fmoke nor fmell, when expofed to a red heat, 

 nor are they blackened ; and they difTolve with effervefcence 

 in acids, without leaving any refidue. They confift, there- 

 fore, of carbonate of lime, cemented together by a fmall 

 portion of animal matter, which is foluble in acids, and 

 therefore refembles gelatine. 



Some patella: from Madeira, examined by Mr. Hatchett, 

 confiftcd alfo of carbonate of hme, but they emitted a fmell 

 like horn, when expofed to a red heat, and left a femi- 

 liquid gelatinous matter behind, when difTolved in acids. 

 They contain, therefore, lefs carbonate of lime, and more 

 animal matter, which is alfo of a more vifcid nature than 

 tliat of porcellaneous flioUs. 



The mother-of-pearl (hells, when expofed to a red heat, 

 crackle, blacken, and emit a ftrong fetid odour. When 

 immerfed in acids, they effervefce at firft ftrongly ; but 

 gradually more and more feebly, till at laft the cmifTion 

 of air-bubbles is fcarcely perceptible. The acids take up 

 only lime, and leave a number of thin membranous fub- 

 ftances, which ftill retain the form of the fliell. From 

 Mr. Hatchott's experiments, we learn that thefe membranes 

 have the properties of coagulated albumen. Thefe fhells, 

 then, are compofed of alternate layers of coagulated albumen 

 and carbonate of limo, beginning with the epidermis, and 

 ending with the laft formed membrane. The animals 

 which inhabit thefe (hells, inrreafe their habitation by the 

 addition of a ftralum of carbonate of lime, fecured by a 

 new membrane. 



Different fhells vary confiderably in the proportion of 

 their conftituents, and in the confiftcncy of the albuminous 

 part. Some, as the common oyfter-ftiell, approach nearly 

 to the patellae, the albuminous portion being fmall, and its 

 confiftence nearly gelatinous ; while in others, as the halyotis 

 iris, the turbo olearius, the real mother-ofnpearl, and a 

 fpecies of frefli-water mufcle, the membranes arc diftinft, 

 thin, compaft, and femi-tranfparent. One hundred parts of 

 mother-of-pearl contain fixty-fix of carbonate of lime, and 

 thirty-four of membrane. Merat-Guillot in Ann. de 

 Chimie, torn, xxxiv. p. 7 1. 



Pearls, or the concretions formed in thefe fhells, refemble 

 them exaftly in ftrufture and compofition. The fubftance 

 confifts of concentric and alternate coats of thin membrane 

 and carbonate of hme. Hatchett, in Phil. Tranf. 1799. 



The bone of the cuttle-fifti was found by Mr. Hatchett 

 to be exaftly fimilar, in its compofition, to mother-of-pearl 

 ftiells. 



Mr. Hatchett compares the porcellaneous fhells to enamel 

 of teeth, (fee Cranium,) and mother-of-pearl ftiells to the 

 bone of teeth, or other bone. (See Bone.) The only dif- 

 ference is, that in enamel and bone the earthy fait is phof- 

 phate of lime, whereas in fliells it is pure carbonate of lime. 



The fliells of the echini, and the crufts of the afterias 

 (ftar-fifh), are made of carbonate, with a fmall quantity of 

 pliofphate of lime ; and a greater or lefs proportion, accord- 

 ing to their hardnefs or flcxibihty, of an animal, gelatinous, 

 or albuminous matter. 



Many of the fubftances which compofc the bafis, or 

 hard part of zoophytes, have the hardnefs and appearance of 

 fhell or bone : others are foft, and belong rather to tjie clafs 

 of membrane or horn. From Mr. Hatchett's admirable 

 differtation in the Philofophical Tranfaftions for i8co, and 

 the experiments of Merat-Guillot in the Annales de Chimie, 

 torn, xxxiv., our knowledge of the chemical cbnftitution of 

 thefe fubftances is derived. 



The hard zoophytes are compofed chiefly of three ingre- 

 dients ; I . An animal fubflance of the nature of coagulated 

 albumen, varying in confiftency, fometimes being gelatinous, 

 and almoft liquid, at others of the confiftency of cartilage ; 

 2. Carbonate of lime ; 3. Phofphate of lime. 



In fome zoophytes the animal matter is very fcanty, and 

 pliofphate of lime wanting altogether ; in others, the animal 

 matter is abundant, and the earthy fait pure carbonate of 

 lime : in fome, there is much animal matter, with a mixture 

 of carbonate and phofphate of lime ; and a fourth clufs is 

 almoft entirely deftitute of earthy falts. Thus we have 

 four claftes ; of which the firft refembles porcellaneous fheUs, 

 the fecond mother-of-pearl fhells, the third the crufts of tlie 

 cruftacea and cchino-derniata, and the fourth hora. 



£. When the madrepora virginca is imincrfed in diluted 

 nitric acid, it cffcrvefces ftrongl-', and is foon dllTolved. A 

 few gelatinous particles float in the folution, which is other- 

 wife colourkfs and tranfparent. Ammonia precipitates 

 nothing, but its carbonate i..rows down abundance of car- 

 bonate of hme. It is compofed, therefore, of carbonate of 

 lime and a little animal matter. The following zoophytes 

 yield nearly the fame refults ; vl%. madrepora miiricata 

 and labyrinthica ; millepora C3;rulea and alcicornis ; and 

 tubipora nuifica. 



2. The madrepora 'rmea eftervefccs in weak nitric acid ; 

 but when all the foluble part is taken up, there remains a 

 membrane, completely retaining the original fhape of the 

 madrepore. The fubftance taken up is ,juie lime. Hence 

 it is compofed of carbonate of lime, and a membranaceous 

 fubftance, which, as in mother-of-pearl ftiells, retains the 

 figure of the madrepore. 



C 2 The 



