420 ANIMAL NATURE OF DIATOME^. 



is an inquiry that refers as well to Navicula as to all 

 the other Diatomese, and we have noticed it already 

 when treating of Cydotella. 



The forms of Naviculce of the three following sections 

 {elliptic^, ffibbcs, nodosa,) greatly resemble those of the 

 Surirellce, differing only by the median aperture. In- 

 stances of vittee occur in N. paradowa. 



The two species referred to the section lunatae belong 

 to two types entirely different. The one {N. ? genujleocd) 

 has the two primary surfaces curved, and is a true 

 Achnanthidea without a stipes ; the other, again {N. 

 lunata), has the secondary surfaces curved, and hence 

 has analogy only to those Synedrm which, exactly under 

 this point of view, we have compared to the Eunotice. 



The sipnatellcB, or sigmoid Naviculce, to the curvature 

 of the secondary surfaces which we see in some Synedrce, 

 unite the habitual lanceolate form of the Naviculce, in 

 which all the eleven are very like each other. The 

 greatest difference is that presented by the primary sur- 

 faces, which are either lanceolate also, or linear. 



Now, considering the Naviculcs collectively in an 

 organological view, we find, especially in the larger forms, 

 very important conditions. Along the primary surfaces 

 run two lines or small canals, terminating at their extre- 

 mities in distinct foramina, as we have already found in 

 all the preceding families. And it is around these fora- 

 mina that the valuable observations of Ehrenberg on 

 N. major {viridis, Ehr., not Kiitzing) demonstrate the 

 currents produced in the ambient liquids, flowing as if 

 they issued by one extremity and entered by the other. 

 In each of the secondary surfaces are three ample perfora- 

 tions, one in the centre and two at the extremities ; and, 

 projecting from the latter, soft bodies, which Ehrenberg 

 supposes analogous to feet, and subservient to motion. 

 And it is also in the before-mentioned species that 

 Ehrenberg saw clearly the ingestion of indigo, and indis- 

 putable movements in the internal organs " connected 

 (with the body) by an irritable hyaline jelly, hence often 



