FIVE GREAT ANIMAL GROUPS. 



123 



ma 



clft 



Fig. 63. 



Fig. 64. 



Now if you will call to mind what I have said in regard to 

 the nature of the connection of these groups with each other, 

 when I compared them to clouds which are more or less merged 

 into each other at their margins, you will understand me when 

 I tell you that these diagrams not only represent the type or 

 symbolical form of each group, but also exemplify the tendency 

 of one group to merge into another. What the meaning of this 

 tendency is, I will make apparent in good time. 



Thus, the Protozoa (figs. 55, 56) have an irregular, hardly 

 defined, digestive cavity, (m, ma, ma 1 , a,) on one side of which 

 is a hollow, (/*, h,) which beats like a heart. The nervous 

 system, according to the most recent researches, (see p. 64,) is 

 a layer of nerve-cells, or threads, (nc, we 1 ,) which lie just beneath 

 the surface of the body. 



In Zoophytd) (figs. 57, 58,) the intestine (m, ma, a] is more 

 evident as a canal than in Protozoa, and, among the highest, 

 the heart (h) is a distinct tube which runs along one side of the 

 body, whilst the nervous system, (en, g, g l , g 2 , c, nc, nc 1 , nc 2 ,) 

 although disposed around the body in an apparently diffuse 

 manner, has its principal parts so regularly posited as to stand 

 in perfect symmetry with the other organs ; thus it has two 

 ganglions, (g, g,) one on each side of the heart, (/,) one also on 

 each side (g 1 , g^) above, and one directly on the median line, (en,) 



Figs. 55, 56, longitudinal and foreshortened views of an ideal Protozoan; figs. 

 57, 58, the same of a Zoophyte ; figs. 59, 60, the same of a Molluscan ; figs. 61, 62, 

 the same of an Articulate; figs. 63, 64, the same of a Vertebrate. The corre- 

 sponding parts of the organization are lettered alike in all. Original. 



