PARAMECIUM CAUDATUM 61 



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 of urea and probably of carbon dioxide resulting 

 from the processes of destructive metabolism. Murexid crys- 

 tals 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 " 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. 

 They 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 preparatory 

 to devouring them. This paralysis is due to a minute quantity 

 of poison contained in the thread; in Paramecium, however, 

 there is no evidence to show that the trichocysts are poisoned. 

 The extrusion of trichocysts may be seen by adding a small 

 quaatity of dilute acetic acid to the medium. 



Nutrition. The main food of Paramecium is bacteria which 

 are always present in infusions. A constant stream of water 

 passes into the mouth and down the gullet, and bacteria carried 

 by the stream are constantly taken into the endoplasm. A 

 gastric vacuole forms at the bottom of the gullet which gradu- 

 ally fills with water and bacteria. It is then carried away from 

 the mouth by the streaming protoplasm (cyclosis) and the proc- 



