50 GENERAL HISTOEY OF THE INFrSORTA. 



condition The identity of this substance with endoclirome is contradicted 



by KUtzing's own experiments, ...... which prove it to be very rich in 



nitrogen : it emits ammonia copiously when decomposed by heat ; and this 

 can only proceed from a substance abounding with it, which such a decom- 

 position compels it to yield up. Nor, on the contrary, do I beheve that there 

 is any weight in the argument from the solubility of its colouring principle 

 in alcohol ; for this is not aproperty pecuhar to chlorophyll, or to any sub- 

 stance of vegetable origin." 



The contents of the frustules are brought into relation with the surround- 

 ing medium through certain pores or fissures, which have been referred to as 

 existing here and there along the sutures between the opposed valves, or 

 othermse between the valves and the interposed connecting membrane. The 

 existence of such openings in the siHcious envelope, and the consequent 

 exposure of the organic internal or primordial membrane in the situations 

 mentioned, have been demonstrated chiefly by the researches of Prof. Smith, 

 who apphes to them the name of 'foramina.^ Thej^ are thus described {op. 

 cit. i. p. 15) : — " Along the line of suture in discifonn or circular fnistules, 

 but more generally at the extremities of the valves only, when the Diatom is 

 of an oblong, hnear, or elongated form, there exist perforations in the silex 

 which permit the surrounding water to have access to the sm-face of the 

 internal cell-membrane. The fonnation of silex seems occasionally to be 

 arrested in the neighboui-hood of these spots ; and the connecting membrane 

 is in consequence either wholly or partially interrupted at such places. Thus, 

 after the internal cell-membrane has been removed by acid, when it often 

 happens that the valves fall away from the connecting membrane, the latter 

 separates into two parts ; and the fi'ustule has in consequence been described 

 as consisting of foui' plates. The interruptions in the silicious epiderm are 

 usually apparent as shght depressions at the extremities of the frustule ; and 

 the appearances they present have been denominated ' puncta' by Mr. Ralfs. 

 In some species these interruptions are more numerous, being found along 

 the entire line of suture, and are often connected with minute canals hollowed 

 out between the silicious epidenn and the internal ceU-membrane, and ap- 

 parently formed by waved flexui'es of the epidermal envelope." The latter 

 constitute the canalicuh heretofore spoken of (p. 45). 



Siebold regarded the longitudinal bands having a double outhne, and ex- 

 tending from the apparent dots or pores at either end of Naviculce to near the 

 centre, to be fissures ; but the account previously given proves this able man 

 to have been mistaken on this point. Like Prof. Smith, however, he con- 

 cluded that the internal membrane was imperforate, and that it served as 

 the medium for the exosmosis and endosmosis attending the function of 

 nutrition. 



Movements of the Diatomeje ; — their character and causes ; — Cilia ; — 

 Circulation of Contents ; — Respiration. — The peculiar movements noticed 

 in many Diatomeae have attracted the observation of all microscopists, and 

 have induced many, especially among the older observers, to receive it as 

 evidence of their animal nature ; but even those who agree on this point 

 are in no better accord among themselves respecting its cause than are those 

 who refer these beings to the vegetable kingdom. 



The power of movement is not confined to those only which are free, but 

 also to concatenate and to some fixed forms, e. g. Synedra, which move on their 

 fixed extremity. There is considerable diversity both in the manner and extent 

 of movement of different species; but in none is it exhibited in an equal 

 degree to that seen in the spores of Algae. " The motion," says Prof. Smith, 

 " is of a peculiar kind, being generally a series of jerks producing a rectilinear 



