820 THE ANIMAL KINGDOM. 



offence such as the rods discharged from the oesophagus in some Infusoria, the 

 trichocysts or miniature thread cells of others (p. 834), a Flagellate (p. 484), and 

 Dinoflagellate (p. 851) of some Sporozoa (p. 864). But it is not certain that the 

 trichocysts in all cases are of use in entrapping prey. The food-material consists 

 either of living or dead animals or plants, and the Protozoon is then said to be 

 holozoic ; or it is organic food-material in solution, and the Protozoon is sapro- 

 phytic ; or finally, in some instances where chlorophyl is present, nutrition appears 

 to take place as in plants, in other words the Protozoon is holophytic. But the 

 presence of chlorophyl need not necessarily lead to holophytic nutrition ; how far it 

 does so, is at present a moot point. The digestion of solid food is effected by 

 contact with the protoplasm, or by a food-vacuole which may be formed either by 

 the inclosure of some water with the food, or by the secretion of a liquid drop 

 round it. There can be little doubt that whenever water is inclosed, the drop is 

 modified by a secretion of the protoplasm ; indeed it may be absorbed and a food- 

 vacuole subsequently secreted. See Krukenberg, Vergleich. Physiol. Vortrage, i. 

 p. 48; Greenwood, 'Digestion in Rhizopods,' Journal of Physiology, vii. 1886. 

 The undigested residue is expelled at any spot when the cell is little differentiated, 

 at a special spot or even by a special aperture when it is highly differentiated (some 

 Infusoria], Excess of nutrition gives rise to reserve material, fat, albumen bodies, 

 starch in some instances, especially in the presence of chlorophyl, glycogen (Barfurth, 

 A. M. A. xxv. 1885, p. 314), or a starch-like substance known as paramylum 

 (p. 843). Special excretory granules or crystals are sometimes found. 



Water may collect in the protoplasm rendering it vacuolate. In most Protozoa 

 its excess is got rid of by the formation of drops or vacuoles, which either collapse 

 slowly or are rhythmically pulsatile, appearing at or near the same spot in rapid suc- 

 cession, and arising either by the increase in size of a single drop, or by the fusion 

 of separate droplets, occasionally irregular in outline, but often disposed in a rosette. 

 The pulsation is slower in marine than in freshwater forms, and it is quickened or 

 the size of the vacuole increased by want of oxygen (Fiszer, Journal R. Micr. 

 Soc. (2), vi. p. 463). The pulsation indeed may take place so suddenly and with 

 such force as to propel the animal onwards (Engelmann, Z. A. i. p. 121). The 

 water expelled may contain granules in suspension, and it has been observed that 

 colouring stains, such as Bismarck brown or aniline blue which do not kill the 

 organism, accumulate in it and are thus removed (Ray Lankester, Encyclopaedia 

 Brit. (ed. ix), xix. p. 836; Q. J. M. xxiv. p. 378). A special vacuolar duct exists in 

 a few instances (some Acinetaria, Vorticellidae). 



A nucleus is seemingly absent in some Proleowyxa, and occasionally in other 

 Protozoa where it is normally present (Gruber, Biol. Centralblatt, iii. p. 580 ; cf. 

 infra, p. 821). In structure it presents great variety. It may be homogeneous, or 

 vesicular, i. e. with a membrane, occasionally much specialised, as in Radiolaria, 

 inclosing a nuclear fluid and variously arranged chromatin. Its division may or 

 may not be accompanied by mitosis. There may be one, or several, or many. If 

 more than one, the increase may be normal to the organism (Gruber, Biol. Central- 

 blatt, iv. p. 710), e.g. in some Infusoria, Actinosphaerium, some Amoebae, and be 

 limited, e. g. to two, or unlimited ; or it is connected directly or remotely with a 

 reproductive phase. Furthermore the many nuclei may be alike in structure and 

 size, or markedly dissimilar, as in the paranuclei of some Acinetaria, the Infusoria 

 and the Dinoflagellate Polykrikos. Occasionally the Infusorian nucleus undergoes 



