THE ANIMAL KINGDOM 3 



bodies consist of a single nucleated cell, the Protozoa, 

 and these are contrasted to all other animals whose 

 bodies are built up of numerous cells, arranged in 

 definite tissues, the Metazoa. Now it is true that 

 certain intermediate forms are known, so-called 

 colonial Protozoa, whose bodies are formed of small 

 colonies of cells, e.g. Volvox, but these organisms 

 do not show any affinity to any known Metazoa, their 

 relationship being indeed rather with the more lowly 

 forms of multicellular plants. 



The most simply organized of the Metazoa are 

 the Coelenterata, including the Jelly-fish, Fresh- 

 water and Marine Polyps, Sea Anemones and Corals, 

 in which the body consists of essentially two cell- 

 layers forming a double cylinder, the one enclosing 

 the other, the outer one, the ectoderm being pro- 

 tective in function, the inner, the endoderm, enclosing 

 a central cavity and exercising the functions of 

 digestion and absorption (Fig. 1). Between the two 

 layers is a structureless membrane, the mesoglaea ; 

 this membrane may be greatly swollen, as in the 

 jelly-fishes, and cellular elements from the ectoderm 

 may invade this jelly and give rise to muscular, 

 nervous, or skeletogenous tissue, but the tissue so 

 formed cannot be regarded as constituting a true 

 cell-layer, as it is a subsequent derivation from the 

 ectoderm and is very frequently absent. The re- 

 productive cells (ova and spermatozoa) may arise in 



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