262 THE PROTOZOA 



embedded in the chromatophores, the centres of the formation of 

 amyloid substances ; and grains of amyloid nature formed by the 

 constructive metabolism of the organism. It is also common to 

 find in the holophytic flagellates a peculiar red spot, or stigma, 

 placed near the anterior end of the body, and probably sensitive 

 to light (p. 205). 



In general, two types of holophytic flagellates can be recognized : 

 first, forms in which, in addition to the organs already mentioned, 

 those pertaining to the holozoic mode of nutrition are also present ; 

 secondly those possessing only the holophytic apparatus. The 

 first type may be regarded as more primitive forms in which the 

 holophytic habit of life has not become so engrained as to exclude 

 any other mode of nutrition ; but a change is still possible, and the 

 organism can combine or vary the holophytic with the holozoic 

 or saprophytic method. In the second type the organism has be- 

 come plant-like, to the complete exclusion of other methods of 

 nutrition ; the body is generally enclosed completely in a firm cellu- 

 lose envelope, allowing diffusion of liquids and gases, but without 

 apertures through which foreign bodies can pass into the interior. 

 Such forms, if they lose their flagellum in the adult state, are classed 

 as unicellular Algae, and the young flagellated individuals are termed 

 " zoospores." The transition from holophytic flagellates to plants 

 is a gradual one, and the border-line is simply fixed by the characters 

 of the " adult," and is therefore as arbitrary as that between Sar- 

 codina and Mastigophora discussed in a previous chapter. 



(c) In saprophytic and parasitic forms no special organs of nutri- 

 tion are present, since the food is absorbed in a fluid condition from 

 the surrounding medium. 



Contractile vacuoles are commonly present in those flagellates 

 which inhabit fresh water. In the more primitive forms the 

 vacuoles empty themselves direct to the exterior. In more highly 

 organized types the vacuoles open into the oesophagus. In 

 Euglena the two contractile vacuoles open into a reservoir- vacuole, 

 which, according to Wager (213), is in open communication with 

 the oesophagus (Fig. 84). 



The nuclear apparatus consists, as a rule, of a single nucleus of 

 vesicular type, with a distinct karyosome. Chromidia are generally 

 absent, but are found in a few cases (Rhizomastigina). 



The relations of the nuclear apparatus and the flagella have been discussed 

 above, and are briefly as follows : 



1. There is a single nucleus with a single centriole, which functions at the 

 same time as centrosome and blepharoplast. Then either (a) the centriole 

 is within, or connected intimately with, the nucleus, in which case the fla- 

 gellum appears to arise directly from the nucleus, as in Mastigina (Fig. 38) ; or 

 (b) the centriole, and the flagellum it gives off, are quite independent of the 

 nucleus, as in Mastigetta (Fig. 40). 



