834 PROTOZOA plasm. They are then spoken of as cell-cuticle if the cell bounds the free surface of a tissue, or as matrix or cell- wall in other cases. The Frotozoon cell-body frequently forms such "cuticles," sometimes of the most delicate and evanescent character (as in some Amoebae), at other times thicker and more permanent. They may give indications (though proper chemical examination is difficult) of being allied in composition to chitin or gelatin, in other instances to cellulose, which is rare in animals and usual in plants. These cuticular deposits may be absent, or may form thin envelopes or in other cases jelly-like substance intimately mixed with the protoplasm (Radiolaria). They may take the form of hooks, tubercles, or long spines, in their older and more peripheral parts free from permeation by protoplasm, though deeply formed in and interpenetrated by it. Such pellicles and cuticles, the deeper layers (if not the whole) of which are permeated by protoplasm, lead insensibly to another category of ectoplastic products in which the material produced by the protoplasm is separated from it and can be detached from or deserted by the proto plasm without any rupture of the latter. These are Shells and Cysts. Such separable investments are formed by the cell-bodies of many Protozoa, a phenomenon not exhibited by tissue-cells. Even the cell-walls of the protoplasmic corpuscles of plant tissues are permeated by that protoplasm, and could not be stripped off without rupture of the protoplasm. The shell and the cyst of the Protozoon are, on the contrary, quite free from the cell- protoplasm. The shell may be of soft chitin-like sub stance (Gromia, &c.), of cellulose (Labyrinthula, Dino- flagellata), of calcium carbonate (Globigerina, <kc.), or of silica (Clathrulina, Codonella). The term "cyst" is ap plied to completely closed investments (" shells " having one or more apertures), which are temporarily produced either -as a protection against adverse external conditions or during the breaking up of the parent-cell into spores. Such cysts are usually horny. Stalks. By a localization of the products of ectoplastic activity the Protozoon cell can produce a fibre or stalk of ever-increasing length, comparable to the seta of a Chaitopod worm produced on the surface of a single cell. EXTOPLASTIC PRODUCTS DISTINCT FROM PROTOPLASM. Without pausing here to discuss the nature of the finest granules which are embedded as a dust-cloud in the hyaline matrix of the purest protoplasm alike of Protozoa and of the cells of higher animals and plants, and leaving aside the discussion of the generalization that all protoplasm presents a reticular structure, denser trabeculae of extreme minuteness traversing more liquid material, it is intended here merely to point to some of the coarser features of structure and chemical differentiation, characteristic of the cell-body of Protozoa. With regard to the ultimate reticular structure of protoplasm it will suffice to state that such structure has been shown to obtain in not a few instances (e.g., Lith- amoeba, Fig. V.), whilst in most Protozoa the methods of microscopy at present applied have not yielded evidence of it, although it is not improbable that a recticular differentiation of the general protoplasm similar to that of the nucleus may be found to exist in all cells. Most vegetable cells and many cells of animal tissues exhibit vacuolation of the protoplasm ; i.e., large spaces are present in the protoplasm occupied by a liquid which is not | protoplasm and is little more than water with diffusible salts in solution. Such vacuoles are common in Protozoa. They are either permanent, gastric, or contractile. Permanent vacuoles containing a watery fluid are some times so abundant as to give the protoplasm a "bubbly" structure (Thalamophora, Radiolaria, &c.), or may merely give to it a trabecular character (Trachelius, Fig. XXIV. 14, and Xoctiluca, Fig. XXVI. 18). Such vacuoles may contain other matters than water, namely, special chemical secretions of the protoplasm. Of this nature are oil-drops, and from these we are led to those deposits within the cell-protoplasm which are of solid consistence (see below). G asfric vacuoles occur in the protoplasm of most Proto zoa in consequence of the taking in of a certain quantity of water with each solid particle of food, such ingestion of solid food-particles being a characteristic process bound up with their animal nature. Contractile vacuoles are frequently but not universally observed in the protoplasm of Protozoa. They are not observed in the protoplasm of tissue-cells. The contrac tile vacuole whilst under observation may be seen to burst, breaking the surface of the Protozoon and discharg ing its liquid contents to the exterior ; its walls, formed of undifferentiated protoplasm, then collapse and fuse. After a short interval it re-forms by slow accumulation of liquid at the same or a neighbouring spot in the protoplasm. The liquid is separated at this point by an active process taking place in the protoplasm which probably is of an excretory nature, the separated water carrying with it nitrogenous waste-products. A similar active formation of vacuoles containing fluid is observed in a few instance s (Arcella, some Amoebae) where the protoplasm separates a gas instead of liquid, and the gas vacuole so produced ap pears to serve a hydrostatic function. Corpuscular and Amorphous Entoplastic Solids. Con cretions of undetermined nature are occasionally formed within the protoplasm of Protozoon cells, as are starch and nitrogenized concretions in tissue-cells (Lithamoeba, Fig. V. cone.}. But the most important corpuscular products after the nucleus, which we have already discussed, are chlorophyll corpuscles. These are (as in plants) concavo- convex or spherical corpuscles of dense protoplasm resem bling that of the nucleus, which are impregnated superfi cially with the green-coloured substance known as chloro phyll. They multiply by fission, usually tetraschistic, independently of the general protoplasm. They occur in representatives of many different groups of Protozoa (Pn>- teomyxa, Heliozoa, Labyrinthulidea, Flagellata, Ciliata), but are confined to a few species. Similar corpuscles ri band-like structures coloured by other pigments are occa sionally met with (Dinoflagellata). Recently it has been maintained (Brandt, 5) that the chlorophyll corpuscles of Protozoa and other animals are parasitic Algas. But, though it is true that parasitic Alg;e occur in animal tissues, and that probably this is the nature of the yellow cells of Radiolaria, yet there seems to be no more justification for regarding the chlorophyll corpuscles of animal tissue-cells and of Protozoa as parasites than there is for so regarding the chlorophyll corpuscles of the leaves of an ordinary green plant. Corpuscles of starch, paramylum, and other amyloid substances are commonly formed in the Flagellata, whose nutrition is to a large extent plant-like. Entoplastic Fibres. A fibrillation of the protoplasm of the Protozoon cell-body may be produced by differentia tion of less and more dense tracts of the protoplasm itself. But as distinct from this we find horny fibres occasionally produced within the protoplasm (Heliozoa) having definite skeletal functions. The threads produced in little cavities in the superficial protoplasm of many Ciliate Protozoa, known as trichocysts, may be mentioned here. Entoplastic Spicules. Needle-like bodies consisting either of silica or of a horny substance (acanthin) are produced in the protoplasm of many Protozoa (Heliozoa, Radiolaria). These are known as spicules ; they may be free or held together in groups and arranged either radially
or tangentially in reference to the more or less spherical