PELAGIC PLANT LIFE 323 



is known as the ring -furrow. It is guarded by projecting 

 borders on the anterior and posterior sides, called ring-borders. 

 It is in this furrow that the second cilium lies and vibrates. 



These principal organs appear in a great variety of shapes. 

 The genus Ceratmm has the anterior end drawn out into a long Ceratmm. 

 horn, which is open at the top ; its posterior end has also nearly 

 always two horn-like projections, which in most species bend in 

 a forward direction. The species of Ceratmm are well supplied 

 with brown pigment granules, and they occur in the upper 

 water-layers, where they constitute an essential part of the plant 



life. The horns must be regarded 

 as suspension-organs, even though 

 the mobility of the cell makes an 

 adaptation of this kind less indis- 

 pensable. We frequently find 

 them, especially in the species 

 of tropical seas, transformed into 

 very consummate suspension - 

 organs. Sometimes they are 

 decidedly long and hair - shaped, 

 sometimes flattened, and in a 

 few species actually terminate in 

 radiating branches. Kofoid has Kofoid. 

 shown that the species of Cera- 

 tmvi can regulate their floating 

 power, and that when, owing to 

 the movement of the water 

 masses, they enter colder or 

 Yio. 2z(y.-PERiDiNiuM DEPREssvM {^\^). warmcr layers of water, they can 



(Schlitt.) .^ r u • u 



shed portions 01 their horns or 

 prolong them at will (see Fig. 227). They have also still 

 another mode of improving their floating power. The cell wall 

 grows in thickness during the whole life of the algse, and 

 simultaneously ribs and pores are constantly developing ; but 

 as soon as the cell gets too heavy, one or even several laminae 

 peel off from the cell armour, and new extremely thin plates 

 take their place. 



The species of Ce^^atiimi are also formed by division, and 

 with them, too, the daughter-cells each retain half of the 

 membrane of the mother-cell, the other half being new. This 

 does not, however, take place within the cell- wall of the mother- 

 cell, and there is therefore no gradual diminution in the bulk of 

 the individual. Sometimes the cells hang together in chains. 



