PROTOZOA. 819 



Radiolaria it is discontinuous, i. e. spicular. During the growth of the cell, the first- 

 formed portions of the skeleton may become inclosed in its substance ; a few 

 Heliozoa and Radiolaria, however, possess a special organic internal skeleton. 



The protoplasm of the body exhibits much variety : it may be of a similar 

 character throughout, or a more or less permanent distinction may exist between the 

 exterior and central parts. It is often described as having a reticulate structure, or 

 more correctly a vesicular, i. e. with more fluid and less refractile particles imbedded 

 in a denser and more refractile matrix. It is sometimes coloured, and the colour 

 may be proper to it, or derived from the food. Pigment may be present, either 

 coloured vacuoles, granules, or corpuscles. Among the last-named, special interest 

 attaches to the chlorophyl bodies which occur in many freshwater forms : whether 

 they are to be considered as proper to the organism, or instances of an association 

 or symbiosis of a green Alga with an animal, similar to that of a yellow Alga 

 with an animal, e.g. an Infusorian (note 3, p. 833), Radiolarian (p. 881). See the 

 account of Symbiosis given pp. 242-4, the authorities quoted p. 245, and the notes 

 on the occurrence of chlorophyl in the account of the different classes (pp. 833, 

 842, 843, 868, 901). 



In the structure of the cell itself attention must be paid to the density of the 

 protoplasm, to the mode of locomotion, of ingestion and digestion of food, to 

 reserve and excretory products, and the nucleus. 



Though the protoplasm is throughout the body of the organism of the same 

 essential structure, yet its density varies, and the products of digestion, foreign 

 bodies, etc., may when it is very fluid be distributed throughout it evenly ; or when 

 its density is greater they may be restricted to a central region (endoplasm), leaving 

 a more or less pronounced clear border (exoplasm) in which contractility is very 

 marked. Some Infusoria indeed possess special fibrils of contractile protoplasm 

 (p. 834), and distinct muscular connections between individuals are met with in 

 some Vorticellids (p. 834). Locomotion is effected by flowing or vibratile extensions 

 of the protoplasm. As to the former, when the protoplasm is not confined by a 

 rigid envelope and is very fluid or very dense, there may be an even flow of the 

 protoplasm as a whole. But as a rule the flow is restricted to partial and change- 

 able extensions or pseudopodia, which become more and more specialised in form, 

 more and more stable as the protoplasm increases in density. The pseudopodia 

 may even lose their locomotor function and be vibratile (Biitschli, Z. W. Z. xxx. p. 

 271; Gruber, Z. W. Z. xxxvi. pp. 461, 462; Id. ibid. xli. p. 212; Btitschli, 

 ' Protozoa/ pp. 123, 672-3, 440). A very large section of Protozoa move solely by 

 vibratile processes, fine cilia or cilia-bundles, stouter and longer flagella, or vibratile 

 membranes (membranellae s. pectinellae). Sometimes both modes of motion may 

 be found in the same individual at different times, e. g. some Flagellata (p. 841 ; 

 cf. p. 845) or in different stages of the life-history, e. g. Radiolaria. 



As to the ingestion of food, it is procured either by a flow of protoplasm 

 inclosing it, by means of pseudopodia, or by the action of vibratile processes, 

 more rarely by special organs like the suctorial tentacles of Acinetaria. When it is 

 carried out by vibratile processes, there is either a special spot, frequently a depres- 

 sion or tube, at which it is taken into the body, or a special organ to retain it like 

 the collar of Choanoflagellata or the vacuolar process of a few Flagellata (p. 842). 

 The pseudopodia in many instances exercise an instantaneously paralysing effect on 

 motile organisms, e. g. Infusoria. Only a few Protozoa possess special weapons of 



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