92 GENEKAL HISTORY OF THE INFUSOEIA. 



formation in different genera, accompanied by great regularity in the indi- 

 vidual species. 



" These variations exhibit themselves in the different modifications of 

 structure which constitute the markings of the valves, aj^pearing imder the 

 form of ribs and nodules, costae, striae, or cellules of an elliptical, circular, 

 or hexagonal outline. A wide comparison of specimens seems to me to prove 

 that these various markings originate in the tendency impressed upon all 

 organized structm-e to develope itself upon the type of the cell, and that the 

 presence of the silicious constituent in the cell-membrane of the Diatom gives 

 a fixedness to this tendency, which, in ordinary cases, is either not discern- 

 ible in the structure of the membrane, or whose effect is obliterated by the 

 coalescence of the softer material which constitutes its substance. However 

 this may be, it ajipears to me certain that the structure of the silicious valve 

 in the Diatomaceae is invariably cellulate, the cellules being more or less 

 modified according to the peculiar requirements of each species, and that no 

 other explanation of their characteristic markings seems consistent with the 

 facts which are established by a carefiil examination and comprehensive know- 

 ledge of Diatomaceous structure. That this explanation does not involve con- 

 siderations at variance wdth the conditions of unicellular vegetable life, will 

 be ob\ious to any one familiar with the stnicture of the sihcious epiderm in 

 the Equisetaceae and Graminaceae, and the distinctly cellulate structui'e of 

 many pollen-grains, while this very presence of silex as a constituent of the 

 cell-wall in the Diatomaceae appears to be wholly unaccoimtable except on the 

 supposition of the vegetable nature of these organisms. In no instance do we 

 find a parallel condition in the animal kingdom (for the secretion of silicious 

 spicula, as an mtemal skeleton, in some of the Spongideae, cannot be regarded 

 as an analogous phenomenon), whereas the vegetable kingdom furnishes us 

 with cases, not merely of the secretion of silex as a vegetable product in the 

 Bamboo, but with frequent instances of its intimate union with cellulose in 

 the membrane which forms the epiderm of the cell, as in the Natural Orders 

 abeady mentioned, in the Palmaceae and others." 



On the nature and mode of deposition of the silex. Dr. Bailey has ad- 

 vanced the statement that the silica in Phytolitharia, as well as in Diatomeae, 

 Polycystineae, and Spongilithes, is not doubly refi'active and polarizing, as 

 Ehrenberg described, and that even the adniitted exception of AracJinoi- 

 discus is not such. The error in supposing it so has originated from the im- 

 perfect removal of the dense carbonaceous tissues which are deposited beneath 

 the silica. 



6. The final argument we have to consider for the animality of the 

 Diatomeae is, that the greater affinity in the chemical composition of the 

 contents, i. e. of the endochrome or gonimic substance, is with j)lants, and not 

 with animals. This argument is certainly based on a nice and very drfiicult- 

 tb-be-determined fact. Meneghini insists on it as important. His remarks 

 have already been given in om- notice of the contents of the frustules, to 

 which we must refer (p. 47), adding here only some supplementaiy obser- 

 vations to fully convey his opinions. '' Finally," he Avrites {op. cit. p. 366), 

 for this is not a property peculiar to chloroj)hyll, " I may add that, if a portion 

 of chlorophyll could be demonstrated in the interior of Diatomeae, this would 

 by no means mvalidate their animal nature ; we might still suppose they had 

 swallowed it for food. As to the oil-globules " w^hich Kiitzing represents, 

 Meneghini considers they may be no more than particles of sarcode, which 

 have an oily appearance ; and he would observe '' that the number and volume 

 of these globides increase considerably after death, and that during life they 

 are situated upon a longitudinal line extending from one extremity to the 



