48o 



TEXT-BOOK OF ZOOLOGY 



Slipper 



about 400 



Animalcule. ( x 

 times. ) 



K. and k. , Nucleus and paranucleus ; W. , 

 cilia — only those at the margin visible ; 

 G., depression in protoplasm, or vestibule, 

 at bottom of which is placed M., the 

 mouth, which is continued into S., the 

 gullet. A few food particles are shown 

 being swept through each. The gullet is 

 shown just open to allow a few food 

 particles, surrounded by water, and to- 

 gether constituting the food vacuole 

 (Nb.), to enter into the interior of the 

 cellular body. Eleven other similar 

 vacuoles are shown ; the undigested 

 materials of a twelfth are being expelled 

 at the anus (A.). p.B.l., Contractile 

 vacuole, which is filled with fluid by six 

 canals arranged in the form of a star ; 

 p.B.2., the second contractile vacuole, 

 which is just opening on the exterior. 



B. Structure. 



(a) As a house is built up out of 

 stones, so the body of all the animals 

 hitherto considered is composed of a 

 large number of very minute bodies 

 called cells (see pp. 253 and 468). 

 The body of the slipper .animalcule 

 (and of all Protozoa), on the other 

 hand, consists only of a single cell. 

 This cell consists of a minute speck of 

 an albuminous substance which has 

 been termed protoplasm, and in which 

 two distinct structures are found, viz., 

 a nucleus and a, paranucleus (K. and k.). 



(b) This unicellular condition also 

 explains the small size of the Protozoa, 

 and the absence in them of organs 

 such as are found in the higher 

 animals, seeing that even a single 

 organ — e.g., a muscle — already con- 

 sists of many thousands of cells. 

 (Protozoa are, in fact, " organisms 

 without organs.") 



(c) As the surface of the body of 

 an infusorian is always somewhat 

 denser than the rest of the proto- 

 plasm, these animals acquire a definite 

 shape (contrast with Khizopoda). 

 This body surface is either entirely 

 (as in slipper animalcule) or partially 

 (as in bell animalcule) covered with 

 vibrating cilia (W.), which serve as 

 locomotor organs, and also for sweep- 

 ing in 



(d) Food. This consists of decay- 

 ing substances, which are found every- 

 where in water in a finely-divided state 

 (see fresh-water mussel), or of minute 

 algse and other microscopic organisms. 



The substances, however, cannot pene- 

 trate through the denser superficial protoplasm. By introducing a little 



