PROTOZOA 3 



ing protoplasm are secreted, and these dissolve the assimilable 

 constituents of the food-bolus. In certain species, however, this 

 phenomenon may be absent (Amoeba blattcea). 



(iii.) Defcecation, the rejection of undigested food-remnants, is 

 accomplished by means of a constant anal part (cytopyge). In the 

 Flagellates and the Rhizopods, however, there is no fixed localization 

 of the act of defaecation. 



(iv.) The contractile vacuole is almost invariably present in the 

 freshwater Protozoa, but is absent in most of the marine and in 

 nearly all the parasitic varieties. With the exception of the Opalina, 

 it is, however, generally present in the Ciliates. It pulsates regularly, 

 becoming gradually filled and then suddenly discharging its contents, 

 and is generally regarded as an organelle both of respiration and 

 excretion. There are certain Amoebae which possess a contractile 

 vacuole, but which should not be included among the true parasites ; 

 they are to be regarded rather as saprophytic commensals. 



The nucleus of the Protozoon is more varied in form and more 

 complicated in structure than was formerly believed to be the case. 

 There is frequently to be found within the nucleus a round inner body 

 called the caryosome. This caryosome reacts more strongly to 

 certain stains than do its surroundings, and in this way it is sometimes 

 of practical service to the scientist. In colour solutions, for instance, 

 the parasitic Amoebae may be distinguished from the leucocytes by the 

 shape of the nucleus, which, in the case of the former, is quite dis- 

 tinctive. It is important to remember that chromatic substances 

 may be released from the nucleus, and may distribute themselves in 

 smaller or larger fragments (chromides) in the plasm. Or, under 

 certain conditions, the entire nucleus may break up into chromides. 

 Such chromides are observed in the parasitic Amoebae and others. 

 In connection with the structure of the nucleus one other point 

 requires mention. A functional double nucleation is frequently 

 observed, which may be of two kinds. In the one case, one of the 

 nuclei is a nucleus of metabolism, and the other a nucleus of repro- 

 duction (Infusoria). In the second case, somatic and generative 

 chromatin are not separated, but are combined in both nuclei, while 

 the division of function is such that it is customary to speak of a 

 " principal " nucleus and a " locomotor " nucleus. An instance is 

 provided by the Trypanosomes, the locomotor nucleus being here 

 termed the " blepharobkst." 



Propagation among the Protozoa is effected as follows : 



(i.) By Division. The word may be used either in its narrower 

 sense of a splitting into two, the daughter-organisms being of equal 

 size ; or it may be used in the sense of budding, the daughter- 

 individuals being of different sizes. 



