8 



ZOOLOGY. 





still firmer intercellular substance ; and when the intercel- 

 lular substance becomes combined with salts of lime form- 

 ing bone, we have 'bony tissue. 



The blood-corpuscles originate from the mesoderm as 

 independent cells floating in the circulating fluid, the blood- 

 cells being formed contemporaneously with the walls of the 

 vessels enclosing the blood. In the invertebrates the blood- 

 cells are either strikingly like the Amoeba in appearance, or 



are oval, but still capable of 

 changing their form. Thus blood- 

 corpuscles arise like other tissues,- 

 J'^;^.^^^^^-M^f^.r^'^'''^^^^V^ that they finally become 



free. 

 Muscular tissue is also composed of cells, which are at 

 first nucleated and afterward lose their nuclei. From being 

 at first oval, the cells finally become elongated and more or 

 less spindle-shaped, forming fibres ; these unite into bundles 

 forming muscles. Each fibre is ensheathed in a membrane 

 called sarcolemma. Muscular fibres may be simple or striated 

 (Fig. 5). The contractility of muscles is due to the con- 

 tractility of the protoplasm 

 originating in the cells forming 

 the fibres. 



Nervous tissue is made up 

 of nerve-cells and fibres pro- 

 ceeding from them ; the for- 

 mer constituting the centres 

 of nervous force, and usually 

 massed together, forming a 

 ganglion or nerve-centre from 



which nerve-fibres pass to the 



periphery and extremities of °®''™° '*• s'- ^> p"'=««^'°s from it. 



the body, and serve as conductors of nerve-force (Fig. 6). 

 Organs and their Functions. — Having considered the 

 different kinds of cells and the tissues they form, we may 

 now consider the origin of organs and their functions. The 

 Protamoeba may be considered as an organless being. In 

 Amoeba (Fig. 11) we first meet with a specialized portion of 

 the body, set apart for the performance of a special function. 



aDglion in the clam, with 



