ANIMAL NATURE OF DIATOMEiE. 435 



hypothetical. Now in opposition to this hypothesis 

 stands the fact of all the Diatomeae destitute of this aper- 

 ture. Instead of it, we find in almost all of them the 

 presence of two terminal perforations of the primary 

 surfaces, always so situated, even in associations of 

 numerous individuals, that they can perform their func- 

 tions freely. The minuteness of these apertiu'es would 

 be adapted to the tenuity of the food they were in- 

 tended to receive, and might, in some degree, explain 

 why nothing can be discerned as to the nature of this 

 food or the conformation of the digestive organs, whilst 

 in other Infusoria, even of smaller dimensions, the sub- 

 standes received can be clearly distinguished. And 

 although the terminal apertures of the secondary surfaces 

 may seem to belong to organs of motion, (as Ehrenberg 

 has described, in some of the larger species, and as appears 

 indicated also by the nature and direction of the move- 

 ments themselves,) still it is a reasonable supposition for 

 any one to believe that the median aperture is subser- 

 vient rather to the generative function. 



38. Frustulia. — Individua navicularia, in substantia 

 gelinea amwpha nidulantia. 



The only character that distinguishes this genus from 

 the NaviculeB is the presence of a mucous envelope. In 

 one of the two species (_F. maritima) the Naviculse are 

 included in various numbers within a distinct cell. 

 Again, in another, {F. salina) the enveloping mucus is 

 amorphous. By the same character Ehrenberg compre- 

 hended also in this genus that N. appendiculata of which 

 Kiitzing had constructed his genus Brachysira. Organo- 

 graphically, the Friistulim show the transition of the free 

 to the included Naviculese. 



39. Berkeleya. — Phycoma gelineum molle basi 

 ylobosum, ramos filiformes naviculis dense aggregatis 

 repletos emittens. 



In this and the two succeeding genera {BapMdogloea, 

 Homceodadia,) there is not merely wanting the primary 



