CHAPTEK YI. 



THE BIOLOGY OF AN ANIMAL {Conti7\ued). 



The Earthworm. 

 Microscopic Stkuctuke or Histology. 



"We have followed the develoj)nient of the one-celled germ 

 through a stage, the hlastula^ 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. 84). 

 As the embryo develops, the cells of these three tissues become 

 differentiated in structure to fit them for difl'erent duties in the 

 physiological division of labor. And when this process of dif- 

 ferentiation is accomplished and the adult state is reached we 

 find six well-marked varieties of tissue, as follows : — 



Principal Tissues of Liimhricus, 



I. EpitheliaL Layer 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 ; hav- 

 ing processes prolonged into slender cords or fibres, bundles of which con- 

 stitute the nerves. 



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

 thelial cells covering the ccelomic surface, but afterwards differentiated 

 into ova and spermatozoa. 



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

 substance, \h^ plasma. 



VI. Connective Tissue. Cells of different shapes, often branched but 

 sometimes rounded, separated from one another by more or less lifeless 

 (intercellular) substance in the form of threads or homogeneous material. 



90 



