PROTOZOA 835 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 FORM OF THE PROTOZOON CELL. Those Proto zoa which have not a differentiated cortical substance, 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 (Ccelentera 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 foul- 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 phenomena 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 -., 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, Radiolaria, Reti- 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 flagella, 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 flagella 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, Radiolaria). Between these extreme forms of "pseudo-
podia " there are numerous intermediate varieties, and the