PELAGIC PLANT LIFE 315 



Pelagic forms usually have thinner cell-walls, and the f 

 characteristic ornamentations on their silicated valves are not 

 so prominent, though in their case too a high magnifying power 

 will nearly always render them visible. The families that are 

 endowed with locomotion organs are very scantily represented, 

 and even amongst the few that are thus favoured, several species 

 make use of them for quite a different purpose, employing them 

 as organs to secrete mucilage and thus keep the cells united in 

 chains. Most of the pelagic diatoms belong to families that 

 lack organs of locomotion, though by way of compensation 

 various types have highly developed suspension organs, which 

 increase their superficies and consequently their friction against 

 the surrounding water-masses. It is possible, too, that these 

 algae are able to reduce excess weight by evolving specifically 

 lighter matter, such as fat, within the cells or air-bladders outside 

 them, but this has not yet been properly investigated. 



The suspension organs, however, have been most carefully 

 studied, especially by Schlitt, who was one of the members of Schutt. 

 Hensen's Plankton Expedition in 1889, and the different cell- 

 forms, with their numerous contrivances for maintaining a 

 floating existence, may be grouped under four heads : — 



(i) TJie Bladder Type.— In these the cell is comparatively large, p'our types of 

 while the cell-wall and protoplasm are merely thin membranes round a suspension 

 big inner cavity which is filled with a cell-fluid of about the same specific °''S'^"^- 

 gravity as sea-w^ater. Among diatoms the best instances of this type 

 are species of the genus Coscinodiscus, whose structure resembles 

 cylindrical boxes, sometimes fairly flat-shaped, and sometimes more 

 elongated and rounded at the top and bottom. In most forms the cell- 

 wall is quite thin, though it is strengthened by means of a fine mesh- 

 work of more or less regular hexagons. One of the biggest, Coscinodiscus 

 rex {Et/unodiscus rex, Antelinine/lia gigas), is over a millimetre in diameter, 

 and is quite a common form in "the warmer parts of the Atlantic (see Fig. 

 215). A series of species with stouter structure, and more distinct orna- 

 mentations on the cell-wall, occur especially in the deeper water-layers, 

 at about the lowermost limit of plant-life (lOO to 200 metres), and 

 belong to a characteristic twilight-flora, of whose existence Schimper 

 became aware during the " Valdivia " Expedition. 



(2) The Ribbon Type. — The surface is enlarged owing to the cell 

 being flattened down into a plane, which is often bent or twisted to a 

 certain extent. Diatoms of this type (see Fig. 216) are scarce. We 

 have, along the coasts especially, a few species with flat cells, which are 

 associated in ribbon-shaped colonies, such as Fragilaria and Climacodimn. 

 The cell-walls of these species are extremely thin, and not of a particularly 

 distinct structure. 



(3) The Hair Type.— The cells are very much prolonged in one 

 direction, or else they are united in narrow, elongated colonies. Diatoms 



