clx Introduction. 



of a ciliated embryo, s. ' plaaula/ is wanting" ; or it may be accom- 

 plished by metamorphosis, complicated with ' alternation of genera- 

 tions.' In this latter case the specialized generative zooid may be 

 either fixed to the asexual coenoecium, or set free as a medusiform 

 zooid. The power of repair and of regeneration after injuries, and 

 from isolated portions of body substance, is very great. 



Sub-kingdom, Protozoa. 



Organisms, which, being ordinarily microscopic and unicellular, 

 rarely have their exterior outlines fixed in definite forms, or their 

 interior parenchyma distinguished by much histological differentia- 

 tion. By the absence of a rigid external envelope the unicellular 

 Protozoon is distinguished from most forms of vegetable life, except 

 the Mycetozoa and the locomotor gonidia of certain Cryptogamia ; 

 and by the absence of histolog-ical differentiation in correspondence 

 with the different vital functions, it is distinguished from most or 

 all higher animal organisms. The greater part, or even the whole 

 of the body of a Protozoon, may consist simply of Protojilasm, 

 s. Cytoplasm, s. Sarcode, which may vary from being purely 

 hyaline to being markedly granular, and from being semifluid to 

 being exceedingly viscid and strongly coherent, but which, chemi- 

 cally, is nitrogenous, and physiologically, contractile. The exterior 

 or cortical layers of the body ordinarily differ more or less in con- 

 sistence from the interior ; they may secrete a calcareous, or agglu- 

 tinate an arenaceous shell; and the internal layers may, in their 

 turn, furnish themselves with a solid support in the shape of an 

 internal skeleton composed of inorganic, siliceous, or calcareous 

 particles, or of organic horny substance. In some cases an exter- 

 nal shell is secreted, of an organic substance resembling chitine. 

 Movement may be accomplished by the contraction of the general 

 mass of the parenchyma, as in Gregarinae ; or the j)rotoplasm may 

 develope cilia, as in Infusoria; or pseudopodia, as in other Pro- 

 tozoa. Nutriment is absorbed in some cases by the general paren- 

 chyma of the body which envelopes the alimentary matter within 

 its own substance, as in Amoehina and Actinophryna; or the entire 

 animal may live immersed in an atmosphere of assimilable albumen. 



