The microscope and its revelations . ers web. According to ,* who has carefully examined its structure, each valveconsists of two layers; the outer one, a thin flexible homymembrane, indestructible by boiling nitric acid; the inner one,siliceous. It is the former which has upon it the peculiarspiders-web-like markings; whQst it is the latter that formsthe supporting framework, which bears a very strong resem-blance to that of a circular Gothic window. The two canoccasionally be separated entire, by first boiling the disks fora considerable time in nitric acid, and then carefully was
The microscope and its revelations . ers web. According to ,* who has carefully examined its structure, each valveconsists of two layers; the outer one, a thin flexible homymembrane, indestructible by boiling nitric acid; the inner one,siliceous. It is the former which has upon it the peculiarspiders-web-like markings; whQst it is the latter that formsthe supporting framework, which bears a very strong resem-blance to that of a circular Gothic window. The two canoccasionally be separated entire, by first boiling the disks fora considerable time in nitric acid, and then carefully washingthem in distilled water. Even without such separation, how-ever, the distinctness of the two layers can be made-out byfocussing for each separately under a l-4th or l-5th in. objec- * Transactions of Microscopical Society, 1st Series, vol. iii, p. 49. DIATOMACE^:—AHACHXOIDISCUS, TRICEEATICTM. 319 tive; or by looking at a valve as an opaque object (either bythe Parabolic illuminator, or by the Lieberkiihu, or by a side Fig. Arachnoidismts Ehrenhergii. light) with a 4-lOths in. objective, first from one side, andthen from the other. 183. Nearly aUied to the preceding in general characters,but differing in the triangular shape of its valves, is theTriceratium ; of which striking form a considerable number ofspecies are met-with in the Bermuda and other Infusorialearths, while others are inhabitants of the existing ocean andof tidal rivers. The T. faviis (Fig. 79), which is one of thelargest and most regularly-marked of any of these, occurs inthe mud of the Thames and in various other estuaries onour own coast; it has been found, also, on the surface oflarge sea-shells from various parts of the world, such as those 320 MICROSCOPIC FORMS OF VE&ETABLE LIFE. of Eip2iopus and Haliotis, before they have been cleaned; andit presents itself likewise in the infusorial earth of Petersburg(). Although the triangular form, when the frustule islooked-at sideways, is that which i
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