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CHAPTER XII 



MICROSCOPIC FORMS OF ANIMAL LIFEPROTOZOA 



PASSING on, now, to the Animal Kingdom, we begin by directing 



our attention to those minute and simple forms which correspond in 



the animal series with the Protophyta in the vegetable (Chap. VIII.) ; 



and this is the more desirable since the formation of a distinct group 



to which the name of PROTOZOA (first proposed in this sense by 



Siebold) may be appropriately given is one of the most interesting 



results of microscopic inquiry. This group, which must be placed at 



the very base of the animal scale, is characterised by the apparent 



simplicity that prevails in the structure of the beings that compose 



it, the lowest of them being single protoplasmic particles or ' jelly - 



specks,' whilst even among the highest, however numerous their 



units may be, these are (as among protophtjtes) mere repetitions of one 



another, each capable of maintaining an independent existence. In 



this there is a very curious and significant parallelism to the earliest 



embryonic stage of higher animals ; for the fertilised germ of any one 



of these first shapes itself as a single cell, and then, by repeated binary 



subdivisions, develops itself into a mo-rula or ' mulberry-mass ' of 



cells, corresponding to the ' multicellular ' organisms met with 



among the higher Protozoa. There is, so far, in neither case any 



sign of that ' differentiation' of organs which is characteristic of the 



higher animals ; but whilst, in the Protozoon, each cell is not merely 



similar to its fellows, but is independent of them, the morula, in 



such as go on to a higher stage, becomes the subject of a series of 



developmental changes tending to the production of a single whole, 



whose parts are mutually dependent. The first of these changes is 



its conversion into a gastrula, or primitive stomach, whose wall is 



formed of a double membrane, the outer lamella, or ectoderm, 1 



being derived directly from the external cell-layer of the morula 



whilst the inner, or endoderm, is formed by the ' imagination ' of 



that layer into the space left void by the dissolution of the central 



cells of the 'morula.' This gastrula-stage, 2 as we shall see hereafter, 



remains permanent in the great group of Ccelentera, though the 



endoderm and ectoderm are separated from each other in its higher 



forms by the development of generate and other organs between 



1 The terms cpiblust and Itijjiotiliisf are generally used by English embryologists 

 in place of the ' ectoderm ' and ' endoderm' used here. 



The gastrula-stage is in a number of cases brought about by a concentric split- 

 ting of the walls of the morula into two layers, and by the appearance at one point 

 nt an orifice which leads into the central cavity ; this cavity is the original segmenta- 

 tion cavity of the morula, and not a fresh cavity, as in ' invagimite gastrulae.' 



