18 



FIRST LESSONS IN ZOOLOGT. 



-A 



A solid central mass constitutes the nucleus (JV). A 

 smaller form is called Cliilodon (Fig. 13). 



The trumpet - animalcule (Stentor) is 

 large enough to be detected with the un- 

 aided eye. This Infusorian attaches itself 

 at one end by a stalk, and builds up a 

 slight tube, into which it contracts when 

 disturbed. The Stentor may be sometimes 

 observed multiplying by self -division. 



The process of self -division takes place in 

 two hours. Fig. 14, /, represents the final 

 -T stage when the two individuals swim away 

 separately, each assuming the original adult 

 - c form, a. 



We have thus seen that in the one-celled 

 animals there is a considerable range of 

 form. Their organs are few and of the 

 simplest kind, and yet primitive as the 

 Protozoans are, they vary 

 greatly in form and in hab- 

 its, and certain kinds which 

 grow in compound assem- 

 blages partially bridge ovor 

 the gap separating the one- 

 celled from the many-celled 



FIG. 12. Parameci- 

 um caudatum. A 

 view from the dor- 

 sal side magnified 

 150 diameters. P, 

 the tail; T, the 

 throat; A, the pos- 

 terior opening of 

 the digestive cavi- 

 ty; Cv 1 , the anter- 

 ior, and Cv, pos- 

 terior contractile 

 vesicles; N, a solid 

 central mass or nu- 

 cleus; V, the large 

 vibratile cilia at 

 the edge of the 

 mouth; F, masses 

 of food. 



A B 



FIG. 13. Chilodon. magnified 600 diameters. A, seen from 

 beneath; B, seen sideways. 



animals. The one-celled animals form a branch of the ani- 

 mal kingdom called Protozoa, from the Greek words protos, 

 first, and zoon, animal, meaning primitive or simplest ani- 

 mals. Tliey differ from all oilier animals in being formed 

 of a single cell. Nature is greatest in her smallest things, 



