ANIMAL NATURE OF DIATOME^E. 425 



hypothetical. Now in opposition to this hypothesis 

 stands the fact of all the Diatomese 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 apertures 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- 

 stances 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. ERUSTULIA. Individua navicularia, in substantial 

 gelinea amorpha nidulantia. 



The only character that distinguishes this genus from 

 the Naviculfe is the presence of a mucous envelope. In 

 one of the two species (F. maritima) the Naviculae 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 Bractysira. Organo- 

 graphically, the Frustulia show the transition of the free 

 to the included Naviculea3. 



39. BERKELEYA. Phycoma gclineum molle basi 

 globosum, ramos jUlformes naviculis dense aggregatis 

 repletos emittens. 



In this and the two succeeding genera (Rapliidogloea, 

 Homceodadia,} there is not merely wanting the primary 



