314 



DEPTHS OF THE OCEAN 



fresh and salt water, and they are found not merely as floating 

 forms, but also along the coasts, some of them attached to the 

 bottom or to other algse and animals ; some are capable of 

 motion, gliding over the mud in enclosed bays or among grains 

 of sand near the seashore. The coast forms, however, are 

 essentially different from the pelagic forms in their structure. 

 Littoral diatoms are apt to have a comparatively thick and 

 extremely silicated cell-wall with the characteristic patterns, 

 ribs, and pores, that have made them such an attractive object 

 of study to amateur scientists. Bilateral symmetry prevails, 

 especially amongst forms that are capable of motion, which are 

 as a rule pointed at the ends like the bows of a boat. Diatoms of 



Fig. 214.— Auxospore-formation of Thalassiosira gravida. 

 a. Showing in the centre a newly-formed auxospore, the old cell-walls still lying outside (-y-) ; b, 

 showing on the left a cell before auxospore-formation, succeeded by an auxospore during its 

 first cell-division, the chain of five cells having originated from an auxospore (-"^-). 



this kind have a highly organised locomotion apparatus, which 

 is differently constructed in the different genera, such as 

 Navicula and Nitzschia. Attached forms show more variation. 

 Symmetry with them depends upon the mode of attachment. 

 LicmopJio7'a and Gomphonema are fastened at one end to a 

 gelatine-like stalk, and their cells are wedge-shaped, narrow at 

 the bottom and widening out towards the top. Others, like 

 Epit hernia, are convex on the one side and straight on the 

 other, the straight side being the one by which they are attached. 

 And there are others again that consist of more or less highly 

 organised and often ramifying colonies, composed of series of 

 cells, or sheaths of mucilage, within which the cells are able to 

 move past one another. 



