404 



GELATIN. 



undulatory movement of the whole body, a 

 mode of progression which in Thethys is mate- 

 rially assisted by the membranous expansion of 

 the mantle placed around the anterior part of 

 the body, which forms a broad veil, and from 

 the muscular fibres contained within it, must 

 necessarily be an important agent in swim- 

 ming. 



Particular secretions. Many of the Gaste- 

 ropoda, in addition to the secretions which 

 have been mentioned, furnish others adapted 

 to peculiar circumstances, and produced from 

 special organs. 



In the Snail and the Slug tribes a slimy 

 mucus is furnished in great abundance from an 

 organ which has been denominated the " sac 

 of the viscosity ;" this is a membranous bag sur- 

 rounding the pericardium, which when opened 

 is found to be divided internally by delicate 

 septa arising from its walls ; from this proceeds 

 a capacious duct, which follows the course of 

 the rectum, to which it is intimately united, to 

 open externally in the neighbourhood of the 

 respiratory aperture. The viscid secretion of 

 this gland spreading over the surface of the 

 foot is most probably an assistant in progres- 

 sion, causing it to adhere more intimately to 

 the surfaces over which the animal crawls. 



Aplysia furnishes three distinct fluids issuing 

 from different parts of the body. The first is a 

 glairy mucus, which exudes in considerable 

 quantities from the surface of the mantle, espe- 

 cially when the creature is irritated. The se- 

 cond is a whitish liquor, which is thick and 

 acrid, and has been reputed venomous; it is 

 emitted in very small quantities, but its smell 

 is strong and highly nauseous: the gland which 

 produces it is a little reniform mass placed near 

 the vulva, close to which is the orifice of its 

 excretory canal. Blainville looks upon this as 

 the representative of a urinary apparatus, but it 

 does not appear to exist in all the species, and 

 is never emitted except when the animal is tor- 

 mented. 



The third secretion is much more abundant 

 than the other two, and is generally of a beau- 

 tiful lake colour, except in Aplysia citrina, in 

 which it is yellow. It is contained in a spongy 

 substance, which occupies all those portions of 

 the little mantle or operculum to which the 

 shell does not extend. All the areolae of this 

 tissue are filled with a purple matter, the colour 

 of which is so intense, that when it is expressed 

 it has a black violet hue, but when mixed with 

 a large quantity of water, imparts to it the co- 

 lour of port wine. This colouring fluid seems 

 to exude through the skin of the mantle, no ex- 

 cretory duct having been found specially ap- 

 propriated to its escape : it is apparently pro- 

 duced from a triangular glandular mass situated 

 in the base of the mantle. 



Several species of Murex secrete a similar 

 fluid, which, like the ink of the cuttle-fish, 

 serves as a defence from attack ; in all cases it 

 is expelled with force, and in such abundance 

 as to colour the water around to a considerable 

 distance. 



There is a species of Limax, (Limax nocti- 

 lucus,) described by M . Orbigny, which produces 



a phosphorescent secretion capable of emitting 

 a light of considerable brilliancy. The luminous 

 organ is a small disc of a greenish colour by 

 day-light, soft in texture, and slightly contractile. 

 The light is only visible when the creature is 

 expanded and in motion. The disc is always 

 covered with a greenish mucus, which, if wiped 

 off, is speedily renewed. It is found to be 

 connected with the generative organs, and ap- 

 pears to be principally useful during the season 

 of love. 



BIBLIOGRAPHY. Swammerdam, Biblia Naturae 

 seu Historia Jnsectorum, fol. 1737. Cuvier, G. 

 Le9ons d'Anatomie Comparee, 8vo. 1799. Ibid. 

 Memoires pour servir a 1'Jiistoire et 1'Anatomie des 

 Mollusques, 4to. 1817. De Blainville, de 1'Orga- 

 nization des Animaux, ou Principes d'Anatomie 

 Comparee, 8vo. 1822. Delle Chiaje, Memorie sulla 

 storia e notomia degli animali senza vertebre del 

 Regno di Napoli. Ferussac, Histoire des Mollus- 

 ques terrestres et fluviatiles, fol. Spallanzani, 

 Opuscoli di Fisica animale e vegetabile, 1776. 

 Reaumur, De la formation et de 1'accroisse- 

 ment des coquilles des animaux tant terrestres 

 qu'aquatiques, in the Memoires de 1'Acad. des 

 Sciences, 1709. He continued the subject in the 

 same work for 1716, under the title of Eclaircisse- 

 mens des quelques difficultes sur la formation et 

 1'accroissement des coquilles. Hatchett, on the 

 chemical Composition of Shells, Phil. Trans. 1799- 

 1800. Beaudant, Memoire sur la structure des 

 parties solides des Mollusques, Annales du Museum, 

 torn. xvi. p. 66. Weiss, M. Sur la progression des 

 Gasteropodes Terrestres, Journ. de Physique de 

 Rozier, An. i. p. 410. Lamarck, Systeme des Ani- 

 maux sans Vertebres, 7 vol. 8vo. 1815-1822. Har- 

 derus, Examen Anatomicum cochleae terrestris do- 

 miportae, Basileae, 1679. 



( T. Rymer Jones.) 



GELATIN (Fr. gelatine; Germ. Leim. 

 Gallerte). This term is applied to an im- 

 portant principle obtained by boiling certain 

 animal substances in water, and filtering or 

 straining the solution, which, if sufficiently 

 concentrated, gelatinises, or concretes into a 

 translucent tremulous mass on cooling, which 

 may be again liquefied and gelatinised by 

 heat and cold. Many varieties of gelatin 

 occur in commerce, of which glue is perhaps 

 the most important : it is obtained by boiling 

 the refuse pieces of skin and hide, and the 

 scrapings and clippings from the tan-yard, in 

 a sufficient quantity of water, till a sample 

 taken out of the boiler forms, on cooling, a 

 stiff jelly; the solution is then strained whilst 

 hot, and run into coolers, where it concretes, and 

 is afterwards cut by a wire into slices, which 

 are dried upon nets. Membranes, tendons, 

 cartilage, horn-shavings, and other similar sub- 

 stances also yield a jelly, which, however, is 

 less stiff and binding than the former, espe- 

 cially when obtained from young animals : 

 size is a jelly of this description. Isinglass, 

 which consists of several parts of the entrails 

 of fish, and especially the sound, &c. of the 

 sturgeon, yields a very pure and tasteless jelly, 

 which is chiefly used for the table; the jelly 

 of calves' feet and hartshorn-shavings is some- 

 what similar. 



As jelly cannot be extracted by cold water, 

 and as we have no direct evidence of its 

 existence in the various substances from which 



