UNITIES OF LIFE 



bium, wood, etc.). -They make up the bodies of the 

 more complex vascular plants, the ferns and flowering 

 plants. 



In the bodies of the tissue-animals we may similarly 

 distinguish two chief groups of tissues, the primary and 

 secondary. The former are phylogenetically and onto- 

 gene'tically older than the latter. The primary tissues 

 of the metazoa are the epitelia, simple layers of cells or 

 forms of tissue directly derived from such (glands, etc.). 

 Secondary tissues, evolved from the former by physio- 

 logical change of work and morphological differentia- 

 tion, are the apotelia; of these "derivative tissues" we 

 may distinguish the three leading groups of connective 

 tissue, muscular tissue, and nerve tissue. These three 

 great groups of tissue in the animal world may be sub- 

 divided, like the plant groups, into lower and higher 

 sub-sections. The coelenteria (gastrasads, sponges, cni- 

 daria) are predominantly built up of epitelia, as are also 

 the phyletically older group of the coelomaria ; in the vkst 

 majority of the latter, however, the great mass of the 

 body is formed of apotelia, and they are subject to the 

 most extensive differentiation. The embryo of all the 

 metazoa consists solely of epitelia (the germ-layers) at 

 first; apotelia are developed from these afterwards by 

 differentiation of the tissues. 



Comparative anatomy distinguishes in the multi- 

 cellular body of the tissue-forming organisms a great 

 number of different parts, which are regularly adapted 

 to discharge definite vital functions, and have been 

 most intricately developed in virtue of the division of 

 labor. They are called "organs" in the stricter sense 

 in opposition to the organella (or organoids) of the pro- 

 tists; the latter have, it is true, a similar physiological 

 purport, but are not (being parts of a cell) equal to 

 the former morphologically. The remarkable efficiency 

 that we find in the structure of the various organs in 



163 



