PEOTOZOA 



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 Protozoon cell-body frequently forms 

 such "cuticles," sometimes of the most delicate and 

 evanescent character (as in some AmoebBe), 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 c'an 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 (Labyrinthnla, Dino- 

 flagellata), of calcium carbonate (Globigerina, (fee), 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. 

 Chsetopod worm produced on the surface of a single cell. 



Entoplastic 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 trabeculse 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- 

 amceba. 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 Noctiluca, 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). 



Gastric 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 instances 

 (Arcella, some Amcebse) 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 (Pro- 

 teomyxa, Heliozoa, Labyrinthulidea, Flagellata, Ciliata), 

 but are confined to a few species. Similar corpuscles or 

 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 Alg». But, though it is true tljiat parasitic Algse 

 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 CUiate 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 



