THE LOWER FORMS OF LIFE. 



27 



to get a very fair knowledge of the structure of the entire group. 

 Fig. 18 is a view of the animalcule as seen when looking down 

 through the microscope upon its back. Observe, first, the mantle of 

 cilia which covers its entire surface. These are its organs of locomo- 

 tion, and also, as just stated, the media by which it forms currents 

 and draws its prey within its grasp. Minute and delicate as are 

 these beautiful organs, my friend, Professor Alhnan, of the Univer- 

 sity of Edinburgh, discovered that the creature has imbedded in 

 its "cortical layer" a vast number of cells, of a peculiar con- 

 struction, called trichocysts, which the animal can burst at will, 

 and from which protrude a great number of extremely fine (even 



d : 



Fig. 18. Fig. 19. 



Fig. 18. Paramxcium Bursaria, dorsal side (after Stein), magnified 300 diameters a a, 

 contractile chambers b, nucleus c, nucleolus d d, particles of food surrounded by a rim 

 of water -e, chlorophylle grains-/, cortical layer, on the outside of which is the 

 u cuticula " or outer covering, bearing (g) the cilia. 



Fig. 19. Diagrammatic section of Paramxtium Bursaria a, the double line represents the 

 cuticula, or secreted outer envelope, carrying the cilia 6, the cortical layer, passing through 

 which the urticating threads are seen. When the animal does not use them they are 

 inclosed in cells termed trichocysts c c, the contractile chambers in the cortical layer 

 d, the mouth e, the "central sarcode," which, being thinner than that of the "cortical 

 layer," is continually circulating round the space it occupies, carrying with it the food. 

 It contains the nucleus, &c., as shown in preceding figures, which is, however, generally 

 attached to the cortical layer. 



when compared with the cilia) threads, which are supposed to have 

 the power of benumbing its prey a power well known to be 

 possessed by these threads in animals higher in the scale. Observe 

 now the shape of the body, and note that all the way round there is 

 a thickened border or edge, which consists first of an outward struc- 

 tureless covering, which is, in fact, an excretion from the body of the 

 animalcule. This is termed the cuticula, and the rest of the dark 

 border consists of sarcode, and is called the " cortical layer" 

 (Fig. 19). This cortical layer gradually becomes softer until it is semi- 

 fluid, and in this condition the sarcode fills up the rest of the body. 



