tetragonal system, with three axes at right angles to each other, 

 two equal and the third longer or shorter. As a simple 

 illustration the Menthaceae may be said to have orthorhombic 

 leaves and a tetragonal stem. 



The essential difference between plants and animals seems 

 to be merely a difference in mode of development and hence of 

 form, in most cases, though not always, correlated with a 

 difference in the type of food and in the manner of its 

 assimilation. 



The primitive cell from which both plants and animals 

 are derived appears to have been ensheathed with an organic 

 deposit, the cell membrane or a thickened derivative from it, 

 and to have contained crystals of various inorganic substances 

 (opaline silica, calcium carbonate, calcium oxalate, calcium 

 phosphate, magnesium carbonate, barium sulphate, etc.) within 

 the cytoplasm. Such cells are represented in various protozoans, 

 in the myxomycètes, in the sponges, in the turbellarians, 

 in the cestodes, in the rotifers, in the tunicates, and, less 

 frequently, in many other animals, but are more typically 

 developed in the raphide bearing cells of plants. 



From a mass of cells of this sort a skeleton may be formed 

 in either of two ways, by increasing the thickness and density 

 of the external organic layer and reducing the cell cavity to 

 a minimum, or by increasing the amount of the inorganic 

 substance, which may be laid down within the cell, deposited 

 in the cell wall, or excreted from it. 



Animals maintain the necessary circulation of liquids about 

 their constituent cells through their own muscular efforts ; 

 to reduce these efforts to a minimum the cell borders must 

 remain as thin as possible so as to afford the maximum surface 

 through which interchange of substances may take place 

 between the cytoplasm and the exterior. Hence the simpler 

 animals for the most part construct external skeletons which 

 may take the form of organic, inorganic, mixed or agglutinated 

 tubes within which they live freely or partially attached, or of 

 organic, inorganic or mixed supports to which, though they 

 form structurally no part of their bodies, they are firmly 

 attached, or a combination of the two. Diffuse internal intra- 

 cellular or extracellular skeletons are also found in the shape 

 of silicious and calcareous spicules, rods, asters, etc., and 



