ii PHYLUM PROTOZOA 67 



therefore, usually bilaterally symmetrical or divisible into equal and 

 similar right and left halves by a vertical antero-posterior plane. 



Some of the lower forms have no distinct cuticle, and are able, 

 under certain circumstances, to assume an amoeboid form (2). 

 The curious genus Mastiyamceba (4) has a permanently amoeboid 

 form, but possesses, in addition to pseudopods, a single long 

 flagellum. It obviously connects the Mastigophora with the 

 Rhizopoda, and indeed there seems no reason why it should be 

 placed in the present group rather than with the Lobosa. Simi- 

 larly, Dimorpha (-5) connects the Flagellata with the Heliozoa : in 

 its flagellate phase (a) it is ovoid and provided with two flagella, 

 but it may send out long stiff radiating pseudopods, while retaining 

 the flagella, or may draw in the latter and assume a purely 

 helizoan phase of existence provided with pseudopods only (&). 



The number of flagella is subject to great variation. There 

 may be one (Fig. 47, l-3\ two (9, 10), three (6\ or four (7). 

 Sometimes the flagella show a differentiation in function ; in 

 Hctcromita, e.g. (Fig'.' 51) the anterior flagellum (ft. 1} only is 

 used in progression, the second or ventral flagellum (jfi. 2) is trailed 

 behind when the animal is swimming freely or is used to anchor 

 it to various solid bodies. 



There are also important variations in structure correlated with 

 varied modes of nutrition. Many of the lower forms, such as 

 Heteromita, live in decomposing animal infusions: they have 

 neither mouth nor gullet and take no solid food, but live by 

 absorbing the nutrient matters in the solution ; their nutrition is, 

 in fact, saprophytic, like that of many fungi. A few live as para- 

 sites in various cavities of the body of the higher animals. One 

 Euglena-like form lives as an mtra-cellular parasite within the 

 cells of one of the lower worms. 



Hccmatococcus (Fig. 48), Pandorina (Fig. 49), Volvox (Fig. 50), and 

 their allies present us with a totally different state of things. 

 The mouthless body is surrounded by a cellulose cell- wall (c.w.\ 

 and contains chromatophores (chr.) coloured either green by chloro- 

 phyll or red by hsematochrome. Nutrition is purely holophytic, 

 i.e. takes place by the absorption of a watery solution of mineral 

 salts and by the decomposition of carbon dioxide. It is, therefore, 

 not surprising that these chlorophyll-containing Flagellata are 

 often included among the Algse or lower green plants. 



Other genera live in a purely animal fashion by the ingestion of 

 solid proteinaceous food, usually in the form of minute living 

 organisms : in these cases there is always some contrivance for 

 capturing and swallowing the prey. In Oikomonas (Fig. 47, S) we 

 have one of the simplest arrangements : near the base of the 

 flagellum is a slight projection containing a vacuole (v.i.)', the 

 movements of the flagellum drive small particles (/.) against this 

 region where the protoplasm is very thin and readily allows the 



F 2 



