GENERAL SKETCH 43 



consisting of innumerable chromatin granules distributed throughout 

 the cell ( Trachelocerca, Chcenia teres, Holosticha flava, H. scute Hum t 

 Tetramitus). (2) The homogeneous nucleus consisting of a single 

 mass of chromatin with a homogeneous structure throughout all 

 stages, and with no trace of reticular substance (many Phytoflagel- 

 lates). (3) Dimorphic nuclei, consisting of a large nucleus called the 

 macronucleus, and a small one, the micronucleus, in the same individual. 

 The former is generally regarded as functional chiefly in vegetation, 

 the latter in conjugation. With the exception of Polykrikos among 

 the Mastigophora, dimorphic nuclei are found only in the Infusoria 

 {Fig. 14). 



The typical form of the nucleus is spherical, although it may be 

 discoid or ellipsoid, or, in the case of the macronucleus, drawn out 

 into various fantastic shapes, of which the horseshoe (Vorticellidae), 

 the beaded {Stentor and Spirostomum, etc.), or branched (Acineta, 

 Dendrosomd) are examples. 1 



4. Organs of Locomotion. 



With very few exceptions, the Protozoa have the power of moving 

 from place to place. The exceptions are found among the para- 

 sitic Sporozoa, although even here there is, in some cases, a peculiar 

 gliding motion. In no adult sporozoon is there a special organ of 

 locomotion, yet the Gregarinida and Haemosporidiida actually move 

 from place to place, although very slowly. In some cases, the 

 motion is due to peculiar peristaltic waves of contraction ; in 

 other cases to the contraction of muscle-like fibrils, the myonemes. 

 An analogous movement is also known in certain flagellates 

 (Euglenidae) and ciliates (Heterotrichida). In the majority of 

 Protozoa, however, movement is accomplished by the activity of 

 special motor organs, which may be either changeable processes 

 (pseudopodia) or permanent vibratile appendages (flagella and cilia). 

 The changeable processes or pseudopodia, found chiefly in the Sar- 

 codina, are sometimes numerous, sometimes few ; when few in number 

 they are usually short, finger-formed, and quick to change in form and 

 appearance by the flowing protoplasmic substance of which they are 

 composed (Fig. I, A, and Fig. 15, A, B)', when numerous, they are 

 fine-pointed, and often sticky, so that when two or more come in 

 contact, they fuse or anastomose (Reticulariida, Fig. 15, C). Again, 

 the pseudopodia may be fine and pointed, but rigid in structure and 

 unchanging in form, a condition brought about by the presence of an 

 axial filament of stiffened protoplasm, which runs down the centre of 

 each pseudopodium (Heliozoa, Radiolaria, Fig. 15, D\ Unlike pseu- 



1 Cf. Chapter VII. for further details concerning nuclei. 



