considerably divergent, but have not attained such a high 

 grade of organisation as is found in the Ciliata. 



So many modifications of form and structure occur in the 

 higher Infusoria, and so many of these are intermediate or 

 transition forms between the different groups, that it is com- 

 paratively easy to imagine the process of evolution, and to 

 trace the course by which common ancestral forms became 

 gradually modified through successive generations into 

 Heterotricha on the one hand, or Peritricha on the other, or 

 were slowly degraded into the Opalinida. Before leaving the 

 Infusoria, it is well to notice the great range of organisation 

 in the group. The difference between such a simple form as 

 one of the Monads, and such a highly differentiated Pro- 

 tozoon as Paramoecium, or Euplotes, or Stentor, or Vorticella 

 is very great. This is indicated in the table by the length of 

 the line from the point of origin to the top of the Peritricha. 

 It is greater than that of any other group of the Protozoa. 



The Gregarinida,* like all parasitic organisms, are diffi- 

 cult to place, as there is always a probability that they have 

 been considerably modified, or even degraded from the ances- 

 tral type, in consequence of their habits. They are placed in 

 the table at the end of a long branch springing from the 

 main stem of the Protozoa, close to the highest Monera, and 

 extending outward and upward so as to reach a point a 

 little above the level of Amoeba, but far from the axis. The 

 length of the line shows the considerable amount of differen- 

 tiation attained by the group f and its somewhat isolated 

 position, while its point of origin indicates the relationship 

 which probably exists with the Monera, There is a similarity 

 with the life-history of Myxastrum,l and the ancestors of the 



* E. van Beneden, Bull, de VAcad. Roy. de Belgique, 2nd ser., T. xxxi. 

 t E. van Beneden, B^ill. de VAcad. Roy. de Belgique, 2nd ser., T. xxxiii ; 

 and Quart. Journ. Mic. Sc, new ser., vol. xii, p. 211. 



\ Haeckel, Studien iiber Moneren, or Huxley's Iiivertebrata, p. 79. 



