130 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



103. The third group contains many exquisite microscopic 

 objects, some of which may be contemplated with fresh admi- 

 ration every time they are submitted to the magnifier. We 

 have no longer simple, linear, or vittate markings, but the cuticle 

 is cellular or areolate. Some are simple and disciform, as 

 Coscinodisciis, or the discs are marked with radiating lines, as 

 Actinocyclus and Actinoptychus ; others angular, as Arti'pJii- 

 tetras anted iluviana, which occurs not only in a fossil state, 

 but recent, in England and Jamaica ; but curious as these 

 are, the crown of all are such genera as Isthmia, (Fig. 7, 6,) 

 and Biddulxjhia, both of which show, in the most admi- 

 rable way, the mode of increase of the frustules. In some of 

 these, every frustule is attached by a short stem to its neigh- 

 bour. Sporangia are rarely formed. As such genera as Cosci- 

 nodiscus and Actinocyclus, &c., abound in the Baltic, they 

 are probably not rare in Great Britain. A few words must 

 still be added respecting the curious Bacillaria imradoxa, 

 the motions of which I have had more than one opportunity 

 of observing. The frustules, which are long and slender, slip 

 over each other, yet so as always to adhere, so that the whole 

 mass is in motion, though at the same time several groups of 

 frustules are moving in contrary directions. The cause of 

 this motion is wholly unknown, but it is most probably mecha- 

 nical, and not vital. It consists of a succession of jerks, the 

 return being almost in the same path as the procession. The 

 motion is more or less perfectly isochronal : — " An obstacle is 

 not evaded, but pushed aside ; or, if sufficient to avert the 

 the onward course, the latter is detained for a time equal to 

 that which it would have occupied in its forward progress, and 

 then retires from the impediment as if it had accomplished its 

 full course.'' — S'inith, 1. c. The same author estimates the 

 motion of different species as follows : — 



Bacillaria paradoxa, ~ inch per second. 

 Pinnularia oblonga, 2^0 j' » 



Pleurosigma strigosum, -L „ „ 



Nitzschia linearis, ^-i-^ „ „ 



Pinnularia radiosa, ^^ „ „ 



As regard^ utility, it is certain that myriads of mollusca feed 



