62 ORGANISMS OF ONE CELL 



Contractile Vacuoles. The contractile vacuole of Amoeba 

 proteus is a single spherical vesicle which moves in the endo- 

 plasm with the other endoplasmic organs. Paramecium is more 

 highly differentiated in this respect by having two vacuoles 

 which are fixed in the cell, and open to the outside by permanent 

 pores in the membrane. One of the vacuoles is in the anterior 

 third of the body; the other in the posterior third. A conspicu- 

 ous feature in regard to them is that special canals feed them by 

 bringing waste matters from all parts. When these canals are 

 full a characteristic radiate structure about the vacuole can be 

 made out with ease. Systole or rupture of the vacuole and 

 diastole or filling are independent in the two organs, but both 

 radiate structures may be seen at the same moment (Fig. 26). 



The waste matters that are collected and excreted through the 

 vacuoles consist, probably, of urea and carbon dioxide resulting 

 from the processes of destructive metabolism. As in Amoeba, 

 murexid crystals have also been demonstrated in these vacuoles 

 showing the presence of uric acid. 



Ectoplasm. The ectoplasm of Paramecium is much more 

 complicated than the endoplasm, and more so than the ecto- 

 plasm of amoeba. It is covered by a lifeless "pellicle/ 7 equiva- 

 lent to the cuticle of higher animals. The membrane or cortical 

 plasm is relatively thick and forms a firm but plastic covering 

 for the cell, by means of which the organism retains a definite 

 form or "morph." The cilia are inserted in it, each cilium 

 taking its origin from a minute basal granule, from the substance 

 of which it is apparently formed. The ectoplasm is further 

 complicated by the presence of peculiar rod-like elements 

 termed trichocysts. When the organism is irritated in any way 

 the material forming these trichocysts is shot out with consider- 

 able force and a network of threads is formed about the cell. 

 The trichocysts thus serve as a means of protection against 

 small enemies which are prevented by the weft of threads from 

 reaching the cell. In some forms of Infusoria similar trichocysts 

 have an offensive as well as a protective function, the minute 

 organisms being able to sting and paralyze other organisms pre- 

 paratory to devouring them. ' This paralysis is due to a minute 



