40 PROTOZOA 



into a distinct digestive cavity. A complex nervous system, of 

 many special cells, with branched prolongations interlacing or 

 anastomosing, and uniting superficial sense-organs with internal 

 centres, is universally developed in Metazoa. All Metazoa fulfil 

 the above conditions. 



But when we turn to the Protozoa we find that many of th( 

 characters evade us. There are some Dinoflagellates (see p. 130) 

 which have coloured plastids, but which differ in no other respect 

 (even specific) from others that lack them : the former may hav( 

 mouths which are functionless, the latter have functional mouths. 

 Some colourless Flagellates are saprophytic and absorb nutritive 

 liquids, such as decomposing infusions of organic matter, possibly 

 free from all proteid constituents ; while others, scarcely different, 

 take in food after the fashion of Amoeba. Sporozoa in the 

 persistence of the encysted stage are very plant-like, though 

 they are often intracellular and are parasitic in living Animals. 

 On the other hand, the Infusoria for the most part answer to all 

 the physiological characters of the Animal world, but are single 

 cells, and by the very perfection of their structure, all due to 

 plasmic not to cellular differentiation, show that they lie quite 

 off the possible track of the origin of Metazoa from Protozoa. 

 Indeed, a strong natural line of demarcation lies between Metazoa 

 and Protista. Of the Protozoa, certain groups, like the Foramini- 

 fera and Eadiolaria and the Ciliate and Suctorial Infusoria are 

 distinctly animal in their chemical activities or metabolism, their 

 mode of nutrition, and their locomotive powers. When we turn 

 to the Proteomyxa, Mycetozoa, and the Flagellates we find that 

 the distinction between these and the lower Fungi is by no 

 means easy, the former passing, indeed, into true Fungi by the 

 Chytridieae, which it is impossible to separate sharply from those 

 Flagellates and Proteomyxa which Cienkowsky and Zopf have 

 so accurately studied under the name of " Monadineae." Again, 

 many of the coloured Flagellates can only (if at all) be dis- 

 tinguished from Plants by the relatively greater prominence and 

 duration of the mobile state, though classifiers are generally 

 agreed in allotting to Plants those coloured Flagellates which in 

 the resting state assume the form of multicellular or apocytial 

 filaments or plates. 



On these grounds we should agree with Haeckel in distinguish- 

 ing the living world into the Metazoa, or Higher Animals, which 



