32 



GENERAL HISTORY OF THE INFUSORIA. 



EiGURE. — There is an immense diversity of figure among the frustiiles, 

 determined chiefly by that of the opposed valves, but in some degree also by 

 the amount of development of the interposed third segment or eingulum 

 (XYI. 23, 24.) This last Mr. Ralfs considers an essential part of every 

 frustule ; but Prof. Smith states it to be a secondary, non-essential element 

 consequent on the growth of the organism, and specially developed .in rela- 

 tion to the process of self- division. When this eingulum or " connecting 

 membrane " is much enlarged prior to fission, the figui'e as viewed on this 

 side is considerably changed, and the apj)earance of a double frastule often 

 occasioned. 



Not a few of the Diatomese are much elongated and narrow, and from pre- 

 senting a wand-like figui-e (IX. 148, 166, 174 ; X. 184, 185), suggested to 

 Ehrenberg the term Bacillaria to designate the family. However, some 

 species are trapezoid, or square, or nearly so (X. 47, 21, 22), others round 

 like pill-boxes (IX. 131, 181; X. 200, 204), whilst others again are 

 almost globular or sj)heroid, owing to the great convexity of the valves. 

 Several genera are boat-shaped, scaphoid, or navicular in figui-e (IX. 139, 

 135 ; XII. 5, 6, 8, 48, 43) ; some are mther oval, egg-shaped, or ovoid ; 

 many resemble thin flattened discs — are discoid (XI. 33, 35, 36, 39); 

 many are wedge-shaped — cuneiform, or cuneate ; a few are triangular (XI. 

 43, 45) ; lastly, some are curved or twisted on themselves, and others assume 

 in certain directions a sigmoid or an undulated figiu^e (IX. 144, 145 ; XV. 11, 

 22, 59, 60). Evenly-curved valves are said to be arcuate, such as those of 

 Eunotia (IX. 165; XYI. 10, 18), and of some species of Cymhella and 

 Nitzschia, whilst the peculiarly-twisted valves of Campylodiscus (XVII. 

 517) are saddle-shaped. In Cymatopleura, again, the surface of the valves is 

 undulated, and when bent rather sharply at an angle on themselves, the 

 valves become geniculate, as in Ach)ianthidium. 



As a rule the frustules of Diatomeae are sjrmmetrical, and consist of two 

 equal and similar halves ; but exceptions to this are found in the Achnantheae, 

 Cocconeidae, and one or two other families (IX. 159). 



Another variety of frustules is described as winged or alate, — the ala being 

 a smooth expansion in the form of a margin (XIII. 5, 6, 7). The alae may 

 arise from the margin, and are then said to be marginal, as in Surirella, or 

 otherwise from the disc, as in Tryhlionella, in which they are called submar- 

 ginal. A further modification of the valves afiecting the figure is exemplified in 

 Nitzschia and AmpMprora, which have a longitudinal elevated ridge extending 

 from one extremity to the other ; such ridges are called keels, and the valves 

 keeled or carinated. In the discoid forms two portions are commonly distin- 

 guishable, viz. the disc and margin or rim (XI. 31, 35, 38), the one at times 

 separated by a distinct line, and often presenting different sculpturing from 

 the other. The disc moreover exhibits occasionally at its centre a promi- 

 nence or elevated thickness called an iimho or boss. In Eupodiscus (XI. 41, 

 42) tubular horns come off from the surface of the valves, and in Triceratium 

 from the angles. 



The extremities of some species, e. g. in Nitzschia and Pleurosigma, are 

 extremely elongated, forming long, filiform, tubular processes ; and in Den^ 

 ticella, Biddulphia (II. 46, 48, 50), and RMzoselenia (Ehi-.), short tubular 

 processes and spines are produced from the surfaces and margins. These 

 processes are commonly simple, but according to Ehrenberg are branched 

 (ramose) in the genus last cited, and in Dicladia and Syndendrium. More- 

 over, very singular hispid and sometimes bifid processes or styles have been 

 noted on the valves of some species of Goniofhecium (Ehr.), recalling by 

 their figure that of the spines on the sporangia of many Desmidieae. Other 



