6 



PROTOZOA 



body of the Protozoon. A similar production of siliceous 

 spicules is observed in the tissue-cells of Sponges. Crys- 

 tals of various chemical nature (silica, calcium carbonate, 

 oxalate, &c.) are also frequently deposited in the protoplasm 

 of the Protozoa, differing essentially from spicules in that 

 their shape is due purely to crystallization. 



General Foem of the Peotozoon Cell. — Those Proto- 

 zoa which have not a differentiated cortical suhstance, and 

 are known as Gymnomyxa, present very generally an 

 extreme irregularity of contour. Their protoplasm, being 

 liquid rather than viscous, flows into the most irregular 

 shapes. Their fundamental form when at rest is in many 

 cases that of the sphere ; others are discoidal or may be 

 monaxial, that is to say, show a differentiation of one 

 region or " end " of the body from the other. Frequently 

 the protoplasm is drawn out into long threads or filaments 

 which radiate uniformly from all parts of the spherical or 

 discoidal cell-body or originate from one region to the 

 exclusion of other parts of the surface. 



These non-corticate Protozoa can take solid particles of 

 food into their protoplasm, there to be digested in an 

 extemporized " gastric vacuole," at any part or most parts 

 of their superficies. They have no permanent cell-mouth 

 leading into the soft protoplasm since that soft protoplasm 

 is everywhere freely exposed. 



The corticate Protozoa have (with the exception of some 

 parasites) one, and in the Acinetaria more than one, de- 

 finite aperture in the cortical substance leading into the 

 softer medullary protoplasm. This is the cell-mouth, — 

 morphologically as distinct from the mouth of an Entero- 

 zoon as is the hole in a drain pipe from the front door of 

 a house, but physiologically subserving the same distinc- 

 tively animal function as does the mouth of multicellular 

 animals. The general form of the body is in these Proto- 

 zoa oblong, with either monaxial symmetry, when the 

 mouth is terminal, or bilateral symmetry, when the body 

 is oblong and flattened and the mouth is towards one end 

 of what becomes by its presence the " ventral " surface. 

 Though the protoplasm is not nakedly exposed in irregular 

 lobes and long filaments in these corticate Protozoa so as 

 to pick up at all points such food-particles as may fall in 

 its way, yet the protoplasm does in most Corticata project 

 in one or more peculiarly modified fine hair-like processes 

 from the otherwise smooth surface of the cell-body. 

 These processes are vibratile cilia, identical in character 

 with the vibratile cilia of epithelial tissue-cells of Entero- 

 zoa. They are essentially locomotor and current-produc- 

 ing (therefore prehensile) organs, and, whilst unable to 

 ingest solid food-particles themselves, serve to propel the 

 organism in search of food and to bring food into the cell- 

 mouth by the currents which they excite. Either a single 

 vibratile filament is present, when it is called a flagellum, 

 or a row or many rows of cilia are developed. 



Constituent cells of the Enterozoa are well known which 

 closely resemble some of the Gymnomyxa or non-corticate 

 Protozoa in their general form. These are the colourless 

 blood corpuscles or lymph corpuscles or phagocytes (Mecz- 

 nikow, 6) which float freely in the blood and ingest solid 

 particles at any part of their surface as do non-corticated 

 Protozoa ; they exhibit a similar irregularity and muta- 

 bility of outline, and actually digest the particles which 

 they take in. The endodermal digestive cells of some 

 Enterozoa (Coelentera and Planarians) are also naked proto- 

 plasmic corpuscles and can take in solid food-particles. 



No tissue-cells are known which present any close 

 parallel to the mouth-bearing corticate Protozoa. The 

 differentiation of the structure of a single cell has in these 

 forms reached a very high degree, which it is not surpris- 

 ing to find without parallel among the units which build 

 up the individual of a higher order known as an Entero- 



zoon. Cilia are developed on such cell-units (ciliated 

 epithelium), but not used for the introduction of food- 

 particles into the cell. In rare cases (the ciliated " pots " 

 of the vascular fluid of Sipunculus) they act so as to freely 

 propel the ciliated cell through the liquid " blood " of the 

 Enterozoon, as the cilia of a Protozoon propel it through 

 water. An aperture in the cortical substance (or in 

 the cuticular product) of a tissue-cell is sometimes to be 

 observed, but is never (?) used for the ingestion of food- 

 particles. Such an aperture occurs in unicellular glands, 

 where it serves as the outlet of the secretion. 



Physiology. 



Motion, — As has just been hinted, the movement of 

 protoplasm, which in the tissue -cells of Enterozoa and 

 higher plants is combined and directed so as to produce 

 effects in relation to the whole organism built up of 

 countless cells, is seen in the Protozoa in a different 

 relation, namely, as subserving the needs of the individual 

 cell of which the moving protoplasm is the main sub- 

 stance. The phenomeiia known in tissue-cells as " stream- 

 ing" {e.g., in the cells of the hairs of Tradescantia), 

 as local contraction and change of form (e.ff., in the 

 corpuscles of the cornea), as muscular contraction, and as 

 ciliary movement are all exhibited by the protoplasm of 

 the cell-body of Protozoa, with more or less constancy, 

 and are intimately related to the processes of hunting, 

 seizing, and ingesting food, and of the intercourse of the 

 individuals of a species with one another and their evasion 

 of hostile agencies. Granule streaming and the implied 

 movement of currents in the protoplasm are seen in the 

 filamentous protoplasm of the Heliozoa, Eadiokria, Eeti- 

 cularia, and Noctiluca, and in the cyclosis of the gastric 

 vacuoles of Ciliata. Local contraction and change of form 

 is seen best in the Amoebae and some Flagellata, where it 

 results in locomotion. Definite muscular contraction is 

 exhibited by the protoplasmic band in the stalk of Vorti- 

 cella, by the leg-like processes of the Hypotrichous Ciliata, 

 and by the cortical substance of some large Ciliata. Cili- 

 ary movement ranging from the vibration of filaments of 

 protoplasm temporarily evolved, up to the rhythmic beat 

 of groups of specialized cilia, is observed in all groups of 

 Protozoa in the young condition if not in the adult, and 

 special varieties of ciliary movement and of cilia-like 

 organs will be noted below. For an account of the con- 

 ditions and character of protoplasmic movement generally 

 which cannot be discussed in the present article the reader 

 is referred to Engelmann (7). 



The protoplasm of the cell-body of the Protozoa is drawn 

 out into lobes and threads which are motile and are used 

 as locomotive and prehensile organs. These processes are 

 of two kinds, which are not present on the same cell and 

 are not capable of transmutation, though there are excep- 

 tions to both of these statements. The one kind are 

 termed " pseudopodia," and are either lobose or filamentous 

 or branched and even reticular (Figs. IV. and IX. ). The Pro- 

 tozoa which exhibit them are sometimes termed Myxopods. 

 The other kind are cilia and fiagella, and are simple threads 

 which are alternately bent and straightened almost inces- 

 santly during the life of the organism.- These Protozoa 

 are termed Mastigopods. "Whilst the cilia and fiagella are 

 permanent organs, the pseudopodia vary greatly in char- 

 acter ; they are in some cases rapidly expanded and with- 

 drawn in irregular form, and can hardly be said to be more 

 than lobose protuberances of the flowing moving mass of 

 protoplasm. In other cases they are comparatively per- 

 manent stiff threads of protoplasm which can be contracted 

 and can fuse with one another but rarely do so (Heliozoa, 

 Kadiolaria). Between these extreme forms of "pseudo- 

 podia " there are numerous intermediate varieties, and the 



