PROTOZOA. 823 



integral parts of the body. Many of the freshwater forms are subject to the attacks 

 of fungal parasites ( Chytrideae) or Bacteria, which have been the cause of erroneous 

 views as to their reproduction. Some of the marine forms, e. g. Radiolaria, are in- 

 habited by symbiotic yellow algae, and in one or two instances by symbiotic diatoms. 

 A few Amoebae appear to harbour symbiotic (?) fungi. 



The classes of Protozoa are for the most part sharply delimited. The position 

 of a few genera is indeterminate, and one group, Proteomyxa, contains organisms 

 which at present cannot find a place elsewhere. Doubts attach to the animal nature 

 of some, e. g. the Flagellate Volvocina. The question, however, as to what constitutes 

 an animal is treated in the General Introduction. 



A subject which presents some difficulty is the mode in which the classes are 

 to be grouped. Biitschli has not yet published a general introduction to his ' Pro- 

 tozoa ' ; and the only writer who has dealt with this division of the Animal Kingdom 

 in its entirety, from a modern point of view, is Ray Lankester in his article 

 * Protozoa' cited p. 824. By him the classes are grouped in two main divisions, 

 Gymnomyxa and Corticata. The essential features of the first-named are (i) that 

 the protoplasm of the vegetative phase of life is naked, i. e. partially or wholly ex- 

 posed to the surrounding medium ; (2) that solid food may be ingested at any spot 

 or at any spot of a large limited area ; (3) that the distinction between exo- and 

 endoplasm sometimes recognised is not a permanent one, exoplasm becoming endo- 

 plasm and vice versa at different times. The Corticata on the contrary have the 

 protoplasm of the body permanently differentiated into two layers, an outer denser 

 cortical substance and an inner more central fluid substance. In this division are 

 included the Acinetaria, Infusoria, Mastigophora, and Sporozoa. 



Certain objections present themselves to this mode of grouping which may be 

 briefly summarised. Whilst it is perfectly certain that in some Gymnomyxa, e. g. 

 Foraminifera, some very fluid Amoebae, the structure of the body is entirely 

 similar throughout, it is by no means so certain that a constant intermixture of the 

 peripheral and central protoplasm is always taking place in some of the more 

 differentiated Amoebae and more especially most Heliozoa. Nor on the other hand 

 is a differentiation into a permanently distinct cortex and medulla to be found in 

 many Corticata : e. g. the majority of Flagellata, some Infusoria, and Acinetaria. 

 It is possible indeed that the degree of distinctness may vary with the state of nutri- 

 tion. Some Flagellata lose their permanent outline and become amoeboid ; some 

 Gymnomyxa, like the Heliozoa, always have a permanent outline, and when they 

 pass into a fiagellula, e. g. the fission products of Clathrulina or the spore of Myce- 

 tozoa, the outline of the body is invariably definite. Indeed certain adult forms, 

 denominated by Biitschli Rhizomastigina (p. 841), may pass at will from one con- 

 dition to another. If the pseudopodia of Gymnomyxa are extensions of the proto- 

 plasm, so are also cilia, flagella, membranellae, and they may even be retracted 

 under certain conditions. There is also no absolute distinction as mentioned 

 above (p. 819) between the two kinds of processes, and sometimes, e.g. in the 

 Heliozoa and some Radiolaria, the pseudopodia are remarkably stable structures. 

 Furthermore what is to be said as to the skeletal structures of both divisions? 

 Gradations appear to exist in Infusoria, for example from a naked condition to one 

 in which there is a somewhat modified superficial stratum of protoplasm, and from 

 this in turn to a close-fitting or detached cuticular structure. Similar gradations 

 may be traced in Amoebina ; and it may be pointed out that nothing can exceed 



