The Animalcules in Water. 117 



called Isthmia and Biddtdphia, inhabit sea-water, and 

 are very beautiful. But in some diatoms the cells are 

 closely joined, and form a nearly straight filament. If 

 the sides of the cells (generally called the " frustules " 

 of diatoms) are not parallel to each other, a curved 

 filament is produced. 



Sometimes diatoms have stalks to which they grow 

 singly, or in a branched arrangement, as seen in Gom- 

 phonema ; others have a mucous or gelatinous cover- 

 ing, in which a number of the frustules lie. Some are 

 quite free, appearing singly, and these are of various 

 shapes oval, triangular, boat-shaped, and round; 

 these last seeming like disks, and to be found (when 

 in their living state) adhering to sea-weeds. 



These free diatoms have a strange power of moving 

 about. Those of the genus Navicula are familiar 

 objects to the microscopic observer. Several of them 

 may sometimes be seen together in the field of view ; 

 they are shaped like the decks of tiny ships, reminding 

 the beholder of the plan of some great sea-fight, where 

 the vessels appear in various directions; and while we 

 look, we notice that they are gliding along, but with 

 no visible means of propulsion. This motion is well 

 described by Mr. Tuffen West, artist of the beautiful 

 illustrations to the late Professor W. Smith's ' ' Synopsis 

 of the British Diatomacese." 



He says, " It is not the rapid meteor-like whirl 

 of the infusorial animalcules, but a gentle gliding in 

 one direction, the distance and the time occupied in 

 traversing it being clearly defined, and readily noted 

 by a good watch ; there is then a brief period of rest, 



