18 THE BEOINNINOS OF LIFE. 



gregarina is found chiefly in the intestinal canal of the earth- 

 worm. The ordinary adult form is shown in the diagram a. 

 Instead of cilia it has at the distal end a small circlet of hooklets. 

 Diagrams b and c show two individuals uniting. They fuse and 

 pass into the encysted condition as at d ; then the protoplasm 

 breaks up into an immense number of spores, as at f, the 

 investment bursts, and each spore develops into a new gregarina. 

 Those earth-worms have my profound sympathy. But while 

 you have been considering these infusors, and perhaps feeling 

 some disgust at the parasitic forms, you may not have realised 

 that here again you are in contact with creatures which have an 

 intimate connection with yourself. The human body furnishes 

 examples of a multitude of infusors, which are part and parcel of 

 us, just as the blood globules and leucocytes are part and parcel 

 of us. The whole of the respiratory tract, the lower parts of the 

 nasal passages, the central canal of the spinal cord, and other 

 parts of the body are lined with cells, which are furnished with 

 cilia, and if detached will swim about by means of their cilia 

 and maintain for a time an independent life, like true infusors. 

 These ciliated epithelial cells are shown in the photomicrograph 

 now on the screen. 



In all the instances which we have considered we have had 

 cells either consisting of naked protoplasm or surrounded by a 

 cell wall of cellulose, the starch-like substance already mentioned ; 

 but in a very large family of microscopic plants, or animals, as 

 some still insist, the diatomaceae, the protoplasm is enclosed in a 

 minute silicious test or shell. These casings consist of double 

 valves of pure silex or flint, and are objects of exquisite beauty, 

 not only from the variety of their forms, but from the mathe- 

 matical accuracy of their shapes and the marvellously minute 

 markings upon them. In the living state they are filled with a 

 yellow or yellowish green colouring matter, and they are endowed 

 with the power of spontaneous movement, the cause of which is 

 obscure, for they are not furnished with flagella or cilia or any 

 other apparent means of locomotion. The variety and beauty of 

 their forms is shown in the photomicrograph, which is a portion 

 of a strewn slide of 150. [Dk. ground.] That is a smaller 

 portion of the same slide which I have taken with dark ground. 

 [Dk. ground triceratium.] That is a triceratium, or triangular 

 diatom, also on a dark ground. [Group.] This photomicrograph 

 is a grouped slide, from which you will see the mathematical 



