.72 GENEKAL HISTOEY OF THE INFUSOBIA. 



dispersed by the surroimding currents or the movements of worms and in- 

 sects, and their detection becomes in consequence more casual and difficult. 

 By far the greater number of the species I have mentioned belong to those 

 genera whose frustules are adherent, or attached by stipes to foreign bodies, 

 or which form continuous filaments or aggregated frondose expansions. ISTot 

 more than four, viz. CycloteUa Kutzingiana, Navicula firma, Amphora ovalis, 

 and Cymhella Pediculus, are to be regarded as free forms : the reason I have 

 just given will account for this cii'cumstance ; and the larger proportion of 

 adherent or frondose species detected in conjugation may doubtless be ascribed 

 to the firmer position conferred upon such forms by the presence of these 

 accessory methods of attachment and adhesion, while the filamentous species, 

 being usually aggregated in considerable masses or entangled amidst the 

 branches of the larger Algae, are also less liable to dispersion." 



Another mode of development, first pointed out by Mr. Ralfs in his early 

 contributions to the history of the Diatomese {A. N. H. 1843), by an internal 

 gemmation or production of cells approaching in physiological features to 

 self- division, appears to prevail in at least some instances. It is alluded to by 

 Prof. Smith, when speaking of the Meridion circidare {op. cit. 7). He met with 

 a variety of frustules, which upon a close examination, especially in a li\'ing 

 state, led him to the conviction '' that the appearance of a double wall of silex 

 is owing to the formation ^\4thin the original frustule of a second perfect cell, 

 instead of the usual mode of division by which the original fnistule is divided 

 into two half-new cells .... In the present case, the central vescicle or cyto- 

 blast becomes enlarged without division, and secretes on its extension two 

 new valves, which are pushed outwards until they lie in close apj)roximation 

 with the original valves. This process is not always repeated ; the usual mode 

 of seK-division again recul^s, and two valves are formed in the interior of 

 this new cell according to the nonnal method. . . .This unusual method of 

 development is not, however, sufficiently constant to warrant the separation 

 of such frastules from the species in which it occm^s, perhaps hardly sufficient 

 to constitute a variety, as frustules in both the ordinary' and abnormal states 

 may be met with in the same gathering and even in the same filament." 



Himantidimn Soleirolii is another species producing internal cells, which 

 Prof. Smith quoted, remarkmg that he had no doubt it is merely an accidental 

 modification of cell-growth, since, in the same filament, cells thus formed may 

 be frequently found along -vsdth others following the normal mode of self- divi- 

 sion. In Odontidium anomalum, this variety is in fact the usual condition 

 of the frustules, and the ordinary mode of self-di\dsion is but rarely to be 

 met with. A remarkable instance of this abnormal development presented 

 itself to Prof. Smith in Achnanthes subsessiUs, in which '-' the formation of a 

 cell interior to the original one had proceeded through several successive 

 stages, and the result is a compound fnistule, consisting of the mother-ceU 

 and a number of included cells, each successive development being embraced 

 by the others pre\iously formed." 



Mr. Ealfs has recently {J. M. S. 1857, p. 14) recurred to the subject of this 

 plan of reproduction, and has found himself obHged to differ from Prof. Smith 

 in some particulars. He writes : " Although it is true that ' we frequently 

 find in the same filament cells thus formed, and others following the normal 

 mode of growth,' as I foimerly showed, yet I cannot agree to Prof. Smith's 

 statement under Bimantidium Soleirolii, that ' there is no doubt of its being 

 merely an accidental modification of cell-growth.' On the contrary, I beheve 

 it to be a reproductive state of the species, and consequently to have a definite 

 and important part in their economy. 



'' Por several years I have attentively watched the circumstances connected 



I 



