254 PRACTICAL LESSONS IN SCIENCE. 



axis cylinder is naked, having no sheath, and sometimes it has 

 only the sheath of connective tissue. 



A group of nerve cells with their branches forms a ganglion. 

 The brain and the spinal cord are simply groups of ganglia. 

 Nerves are made up of nerve fibers; one set serves to connect va- 

 rious sense organs located in the outer surface of the body with 

 the brain and spinal cord, and another set connect the brain and 

 spinal cord with the muscles. 



The lowest organisms consist of a single cell ; the body is the 

 protoplasm and the cell membrane is the skin. The skin absorbs 

 nutriment and removes waste, the protoplasm assimilates 

 material and grows. But as the organism grows, the body in- 

 creases in three directions, while the skin expands only in two di- 

 rections, so that, finally, the area of the skin is relatively too small 

 to furnish material for the growth of the body. Thus the size of 

 a cell or a one-celled animal is limited. If the organism increases 

 in size it does so by dividing into several cells which arrange 

 themselves so as to give the largest extent of surface, eventually 

 forming a cup-shaped organism with an outer and inner surface. 

 In the outer layers organs of motion and sensation, of respira- 

 tion and excretion are developed, carrying on the animal func- 

 tions and some of the vegetative ; while the inner layers or cavity 

 receives -and digests the food. Thus with an increase of size 

 comes a greater complexity of organism, and the essential char- 

 acteristics of the animal. 



The more highly developed the organs, the more complete the 

 division of labor, the better the work done by each organ and the 

 higher and more perfect the life of the organism. The organs are 

 not only correlated in size but in form, so that from one organ a 

 good idea of the size and form of the organism maybe made out. 



The digestive apparatus, at first a mere cavity, then a tube 

 which at length became separable into three parts, the fore part 

 for the reception of food, the middle part for its digestion, and 

 the hind part for the expulsion of the indigestible portions. 

 Then a mouth with masticatory or sucking organs and salivary 

 glands is developed at the fore end of the tube, and from the fore 



