202 PKOr. HUXLEY ON THE 



modify any conclusious whieli may be based upon our present 

 imperfect acquaintance with the facta of development ; and, in 

 many cases, it is impossible to do more than suggest the conclu- 

 sions towards which these incomplete data tend. 



Among those animals which are lowest in the scale of organi- 

 zation there is a large assemblage, which either present no differen- 

 tiation of the protoplasm of the body into structural elements ; 

 or, if they possess one or more nuclei, or even exhibit distinct 

 cells, these cells do not become metamorphosed into tissues — are 

 not histogenetic. In all other animals, the first stage of develop- 

 ment is the differentiation of the vitellus into division-masses, or 

 llastomeres, which become converted into cells, and are eventually 

 metamorphosed into the elements of the tissues. For the former 

 the name Peotozoa may be retained ; the latter are coextensive 

 with the Metazoa of Haeckel. 



I. The Peotozoa. 



The movements of the body are effected either by pseudopodia 

 or by cilia, which latter may either be small and numerous, or 

 long and single, and at most two. When pseudopodia are the only 

 instruments of progression, the animal maybe termed a myxopod; 

 when numerous cilia, a trichopod ; when single or double flagelli- 

 form cilia, a mastigopod. 



Among the Protozoa, two groups are distinguishable: — 1. The 

 Monera ; 2. The Endoplastica. 



1. The Monera. — There is no " nucleus." Our knowledge of 

 these forms and of their relations is largely due to Haeckel, who 

 has shown that several of them present a remarkable alternation 

 of conditions. Thus, Protamosha is a myxopod which may become 

 encysted, and, in that condition, divides into several portions which 

 are set free and resemble the pai-ent, or are myxopods. Proto- 

 monas is a mastigopod which becomes encysted, divides, and gives 

 rise to myxopods, which subsequently become converted into, mas- 

 tigopods. Myxastrum is a myxopod which becomes encysted, di- 

 vides, and the products of division become enclosed in ovoid cases, 

 whence they emerge as myxopods. Vampyrella is a myxopod 

 which devours Gomplionema and other . stalked Diatoms, encysts 

 itself on their stalks, divides, and gives rise to new myxopods. 

 In Protomyxa^ the primitively independent myxopods unite into 

 Plasmodia. Although our knoAvledge of the structure of the soft 

 parts of the Foraminifera is imperfect, and the case of Gromia sug- 



