26 CHARLES DARWIN 



united into a perfect little sphere, which occupied the posi- 

 tion of the septum at one end of the now quite hollow case. 

 The formation of the granular sphere was hastened by any 

 accidental injury. I may add, that frequently a pair of these 

 bodies were attached to each other, as represented above, 

 cone beside cone, at that end where the septum occurs. 



I will add here a few other observations connected with 

 the discoloration of the sea from organic causes. On the 

 coast of Chile, a few leagues north of Concepcion, the Beaglt 

 one day passed through great bands of muddy water, exactly 

 like that of a swollen river; and again, a degree south of 

 Valparaiso, when fifty miles from the land, the same appear- 

 ance was still more extensive. Some of the water placed 

 in a glass was of a pale reddish tint; and, examined under 

 a microscope, was seen to swarm with minute animalcula 

 darting about, and often exploding. Their shape is oval, 

 and contracted in the middle by a ring of vibrating curved 

 ciliae. It was, however, very difficult to examine them with 

 care, for almost the instant motion ceased, even while cross- 

 ing the field of vision, their bodies burst. Sometimes both 

 ends burst at once, sometimes only one, and a quantity of 

 coarse, brownish, granular matter was ejected. The animal 

 an instant before bursting expanded to half again its natu- 

 ral size; and the explosion took place about fifteen seconds 

 after the rapid progressive motion had ceased: in a few 

 cases it was preceded for a short interval by a rotatory 

 movement on the longer axis. About two minutes after any 

 number were isolated in a drop of water, they thus perished. 

 The animals move with the narrow apex forwards, by the 

 aid of their vibratory ciliae, and generally by rapid starts. 

 They are exceedingly minute, and quite invisible to the 

 naked eye, only covering a space equal to the square of the 

 thousandth of an inch. Their numbers were infinite; for 

 the smallest drop of water which I could remove contained 

 very many. In one day we passed through two spaces of 

 water thus stained, one of which alone must have extended 

 over several square miles. What incalculable numbers of 

 these microscopical animals ! The colour of the water, as 

 seen at some distance, was like that of a river which has 

 flowed through a red clay district; but under the shade of 



