ANIMAL NATURE OF DIATOME^E. 459 



which penetrates within them by endosmosis through the 

 double wall dividing their cavity from that of the pro- 

 ducing cell. When these vesicles become detached, they 

 either maintain a life of their own, as in the epidermis 

 and its productions, or they burst and pour out their 

 contents. If such be the origin of the gelatinous sub- 

 stance enveloping the Schizonemese, we must allow that 

 the secerning surface is either external or internal to 

 some particular organ. And these little cells, invisible 

 from their minuteness or tenuity, may continue to live, 

 or at least to pour out their contents, before or after 

 they issue from the secerning organ. But all this is 

 hypothetical, and wanting in any support of observed 

 facts. Again, considering the history of the successive 

 development of Diatornaceae, it is more probable to 

 suppose the possible permanence, extensibility, and pro- 

 gressive growth of one of the embryonal tunics, of which 

 we have numerous examples, were there no other, in all 

 animals that undergo metamorphoses. 



The Schizonemea3 and the other seven genera (Navicula, 

 Ampldpleura, Ceratoneis, Stauroneis, Amphiphora, Am- 

 phora, Diadesmis), before mentioned, constitute the great- 

 family of Naviculeae. The Naviculeas, says Kiitzing, when 

 treating of their affinities, very much resemble individuals 

 of the preceding families, with which they were formerly 

 confounded ; but they are to be distinguished as well by 

 the central aperture of the two lateral surfaces as by the 

 regularity and symmetry both of these and the primary 

 surfaces. He says, on the other hand, that this aperture 

 is frequently absent, especially in the Schizonemeae, or at 

 least escapes observation by its minuteness. Now I add 

 that we have this same regularity and symmetry of form 

 in the Symdrce, and, with few exceptions, in all the family 

 of Surirelleae. So that there is good reason for inquiring 

 what essential character remains to distinguish the 

 Naviculea?? The consideration before adduced with re- 

 gard to the origin and nature of the investing substance 

 in Schizonema would seem to me to furnish some reason 



