34 PKOTOZOA [CH. 



are not killed by drought. When dry they are blown about, and so 

 appear in infusions. In infusions Bacteria appear first, then Flagel- 

 lata, and finally Ciliata (see page 35). 



Many Flagellata are devoid of a pharynx altogether, and are 

 without chlorophyll; they subsist on the nutritive substances which 

 are dissolved in the fluid in which putrefying matter is soaking. 

 These are reckoned as animals on no very good grounds ; for it is 

 well known that plants can lose their green matter when they can 

 get the materials of protoplasm without building it up from carbon 

 dioxide. How entirely arbitrary the decision is is best shown 

 by the fact that many forms are claimed by both botanists and 

 zoologists. For this reason it is convenient to have a name which 

 denotes simply a living thing without prejudging the question as to 

 whether it is an animal or a plant. Such a v name is supplied by the 

 word organism, which is frequently used. 



The Flagellata, or Mastigophora as they are sometimes called, 

 exhibit a very wide range of structure and the type described gives 

 a very limited idea of the group as a whole. Many of them are 

 parasitic in other animals and have attracted to themselves a great 

 deal of interest in recent years because it has been proved that they 

 are pathogenic, i.e. causers of disease. A very rapid survey of the 

 principal sub-divisions may be given here. The Rhizomastigina 

 are a curious group of forms intermediate between the Amoebidea 

 and other Flagellata; they are devoid of visible myonemes or a per- 

 manent cuticle, but possess the power of emitting pseudopodia. 

 The Silicoflagellata have a single flagellum and possess a spherical 

 perforated shell of silica. Through one aperture of this the single 

 flagellum is protruded. The Dinoflagellata possess a thick 

 cuticle or rather external shell consisting of platelets of cellulose. 

 They possess bodies containing a brown pigment which acts like 

 chlorophyll and live entirely as plants, and on the surface of 

 each there is an antero-posterior and a transverse groove (8<Vos) 

 in each of which a flagellum lies. The largest of these (Noctilucd) 

 is just visible to the naked eye. It possesses large vacuoles which 

 enable it to float and thus render locomotor organs unnecessary. 

 The longitudinal flagellum is minute and the transverse one is 

 changed to a short thick tentacle lying in a shortened trans- 

 verse groove. Noctiluca, like most Dinoflagellata, is marine and 

 causes the diffused phosphorescence of the sea, in which it is some- 

 times present in such numbers as to make the water look like 

 soup. Noctiluca is sometimes made the type of a special division 

 termed the Cystoflagellata. The Choanoflagellata possess a 



