CLASSIFICATION OF THE PROTOZOA 27 



The nucleus plays the same part in the life of the single celled organisms as it 

 does in the cells of the Metazoa and Metaphyta. It appears to influence in a certain 

 manner all, or at least most, of the processes of life, such as motility, regeneration, 

 growth, and generally also digestion. Its principal influence, however, is exercised 

 in the propagation of the cells, as this is always brought about by the nucleus. 



The PROPAGATION of the Protozoa is effected either by division or by means of 

 direct budding. In division, which is preceded by direct or indirect (mitotic) 

 division of the nucleus, the body separates into two, several, or even a great many 

 segments. In this process the entire substance of the body is involved, or a small 

 residual fragment may be left, which does not undergo further division and finally 

 perishes. In the budding method of multiplication a large number of buds are 

 formed, either on the surface or in the interior of the organism. Where divisions or 

 buddings follow one another rapidly, without the segments separating immediately 

 after their production, numerous forms develop, which are often unlike the parental 

 forms, and these are termed swarm spores or spores. Divisions imperfectly 

 accomplished lead to the formation of protozoal colonies. 



Sometimes encystment 1 takes place previous to division. Frequently, also, 

 sexual processes appear, such as the union of two similar (isogamous) or dissimilar 

 (anisogamous) individuals. In the latter case sexual dimorphism occurs, with the 

 formation of males (microgametes) and of females (macrogametes). The union may 

 be permanent (copulation), the process being comparable with the fertilisation of the 

 ovum by a spermatozoon. On the other hand, attachment may be transient (con- 

 jugation) when, after the exchange of portions of the nucleus, the couple separate, 

 to multiply independently of each other. Sometimes there is an ALTERNATION OF 

 GENERATIONS, as there may be several methods of propagation combined in the 

 same species, either direct multiplication, conjugation, or copulation being practised ; 

 the different generations may thus, in certain cases, be unlike morphologically. 



Protozoa inhabit salt water as well as fresh water ; they are also found on land 

 in very damp places, and invade animals as parasites. 



CLASSIFICATION OF THE PROTOZOA. 



Class I. Sarcodina (Rkizopoda}. Protozoa, the body substance of which forms 

 pseudopodia ; many of them are capable of developing chitinous, chalky, or siliceous 

 coverings or skeletal structures, which, however, permit the protrusion of the 

 pseudopodia either over the entire periphery or at certain points. They possess one 

 nucleus or several. 



Order i. Amcebina (Lobosa) naked or with a simple shell, sometimes formed 

 of a foreign substance ; the pseudopodia may be lobose or finger-shaped ; 

 there may be a contractile vacuole ; generally only one nucleus. They 

 live in fresh or salt water, in the soil, and also parasitically. 

 Order 2. Foraminifera (Reticularia). Mostly provided with a calcareous 

 shell, usually consisting of several chambers, and allowing the protrusion 

 of the pseudopodia either at the periphery or only at the opening. The 

 pseudopodia are filamentous and frequently anastomosed ; there is no 

 contractile vacuole ; there are usually several nuclei. Mostly marine. 

 Order 3. Heliozoa. Naked, or with a chitinous or simple radial siliceous 

 skeleton ; the pseudopodia are filamentous, and are frequently supported 



1 Independently of propagation, many protozoa protect themselves from death by encyst- 

 ment when the water in which they are living dries up ; in this condition the wind may carry 

 them over wide tracts of land. 



