CHAPTER IX. 



THE BIOLOGY OF AN ANIMAL (Continued). 

 THE EARTHWORM. 



MICROSCOPIC STRUCTURE OR HISTOLOGY. 



have followed the development of tbe one-celled germ 

 through a stage, the llastula^ in which it consists of a mass of 

 nearly similar cells out of which the various tissues of the adult 

 eventually arise. The first step in this direction is the differen- 

 tiation of the germ-layers or three primitive tissues (p. 14:8). As 

 the embryo develops, the cells of these three tissues become differ- 

 entiated in structure to fit them for different duties in the physi- 

 ological division of labor. And when this process of differen- 

 tiation is accomplished and the adult state is reached we find six 

 well-marked varieties of tissue, as follows : 



PRINCIPAL TISSUES OF Lunibricus. 



I. Epithelial. Layers of cells covering free surfaces. 



(a) Pavement Epithelium. Cells thin and flat, arranged like the 



stones of a pavement. 

 (6) Columnar Epithelium. Cells elongated, standing side by side, 



palisade-like, 

 (c) Ciliated Epithelium. Columnar or cuboid, and bearing cilia. 



II. Muscular. Cells contractile and elongated to form fibres. Often 

 arranged in parallel masses or bundles. 



III. Nervous. Cells pear-shaped or irregular, with large nuclei; having 

 processes prolonged into slender cords or fibres, bundles of which constitute 

 the nerves. 



IV. Germinal. Including the germ-cells. At first in the form of epi- 

 thelial cells covering the coelomic surface, but afterwards differentiated 

 into ova and spermatozoa. 



V. Blood. Isolated cells or corpuscles floating in a fluid intercellular 

 substance, the plasma. 



VI. Connective. Cells of different shapes, often branched but some- 

 times rounded, separated from one another by more or less of lifeless (inter- 

 cellular; substance in the form of threads or homogeneous material. 



