Navicula serians, N. rhomboides, and Pinnularia gibba. 171 



their tails, and also move round in much the same manner, 

 which must be effected by a portion of protoplasm outside their 

 fibrous cell-skeleton. And the concatenated divisions of Oscil- 

 latoria princeps can move in a body backwards and forwards in 

 their sheath, which, like that of Schizonema among the Diato- 

 mese, is evidently secreted from their surface. Indeed, innu- 

 merable instances might be cited in support of the view that in 

 all these organisms, where there is less discovered to support it 

 even than in the Rhizopoda, there is, including the Diatomese, 

 a communication between the interior and exterior of their cells, 

 formed of apertures, however small, which enables the more 

 subtile parts of the protoplasm to obtain connexion with the ex- 

 ternal world. It is these apertures which M. Garreau (Annals, 

 vol. x. 1862) has asserted to exist in the vegetable cell, and 

 which, as I have before stated, when confirmed, will afford a 

 satisfactory explanation of all that class of phenomena which, 

 but for the establishment of this link, still incline many to attri- 

 bute them to direct physical causes, instead of regarding them 

 as induced indirectly through the influence and government of 

 vital or instinctive agency. One might say that there was no in- 

 stinctive agency evinced in a Diatom attaching a particle of foreign 

 matter to its surface, if it had not at the same time the power 

 of retaining or casting it off, or if it adhered to every other par- 

 ticle that came in contact with it, which is not the case. 



Indeed several microscopists, among whom are Ehrenberg, 

 Siebcld, Focke, and Wenham, have asserted positively that 

 motor organs exist outside the frustule of the Diatomese, in the 

 form of pseudopodia and cirri, cilia, exsertile and retractile 

 feet, and an undulating membrane respectively ; and in the 

 subgroup of Rhizopoda to which I have already alluded, viz. the 

 Acinetina, wherein vibratile cilia or tentacula can be projected 

 from the surface, or retracted, as required, with the appearance 

 of extemporization, I have observed all this to take place from 

 a visible sarcode ; but I have never seen in the Diatomese more 

 than indications of the presence of such an organ externally, 

 which, like the transparent portion of the cortical or non- 

 granular ectosarc of some Amoebce, presents all the movements of 

 cyclosis when a particle of foreign matter happens to be attached 

 to it, even without itself being visible. 



Returning, then, to the question of impregnative generation 

 in the Diatomese, it seems to me that, being so closely allied to 

 the Rhizopoda in their organization, they might be inferred, by 

 analogy, to follow the same mode of producing an impregnated 

 generation as JDifflugia. That this mode has been demonstrated 

 I by no means wish to assert; but observations on the subject, 

 made subsequently to those published in my last communica- 



