CHAPTER VII. 



DIFFERENTIATIONS AMONG THE OUTER TISSUES OF 

 ANIMALS. 



291. THE outer tissues of animals, originally homo- 

 geneous over their whole surfaces, pass into a heterogeneity 

 which fits their respective parts to their respective conditions. 

 So numerous and varied are the implied differentiations, that 

 it is impracticable here to deal with them all even in outline. 

 To trace them up through classes of animals of increasing 

 degrees of aggregation, would carry us into undue detail. 



Did space permit, it would be possible to point out among 

 the Protozoa, various cases analogous to that of the Arcella; 

 which may be described as like a microscopic Limpet, having 

 a sarcode body of which the upper surface has become horny, 

 while the lower surface with its protruding pseudopodia, 

 retains the primitive jelly-like character. That differentia- 

 tions of this kind have been gradually established among 

 these minute creatures through the unlike relations of their 

 parts to the environment, is an inference supported by a 

 form which, while the rest of the body has a scarcely dis- 

 tinguishable coating, " agrees with Arcella and Difflugia in 

 having the pseudopodia protrusible from one extremity only 

 of the body." 



Many parallel specializations of surface among aggregates 

 of the second order might be instanced from the Coelenterata. 

 In the Hydra, the ectoderm presents over its whole area no 

 conspicuous unlikenesses ; but there usually exist in the 

 hydroid polypes of superior types, decided contrasts between 



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