252 PRACTICAL LESSONS IN SCIENCE. 



occurring in the blood of the invertebrates, or lower animals, and 

 some of those found in the blood of vertebrates, are colorless, ir- 

 regular in form, having the capacity of movement. They can 

 push out processes and draw them in again, by which means they 

 change position. They originate in the lymphatic glands, and 

 pass into the blood with the lymph. Others found abundantly 

 in the blood of vertebrates have a regular form usually coiitam- 

 inga reddish pigment, haemoglobin, which gives color to the blood 

 and is an important agent in the process of respiration. The ova 

 and spermatozoa, active agents in the process of reproduction, are 

 free cells differing widely in form and size. 



The tissue covering the free surfaces of the body, whether ex- 

 ternal or internal, is called epithelium. Thecells forming thistissue 

 are cylindrical, ciliated, or flattened, making cylindrical, ciliated 

 or pavement epithelium. The cells of the lower layers of this 

 tissue have a semifluid consistency, and are active, continually 

 multiplying by division and growing, while those of the upper 

 layer are less active and more firm in substance, which, finally los- 

 ing their vitality, are thrown off, to be replaced by others from 

 the layers below. 



On the outer surface of the body the upper cells of thistissue, 

 as thick horny layers form claws, nails and hoofs, they form 

 the leathery skin of insects, and with a little calcareous matter 

 they constitute the hard shell or outer covering of the craw- 

 fish and lobster. Feathers, hairs, bristles, scales and the horns 

 of some animals, are the outgrowths of the tissue covering the 

 body. 



The epithelium covering the internal surfaces of the body se- 

 cretes a fluid which keeps the tissue moist, and facilitates the 

 movements of objects over its surface. Sometimes tubular, flask- 

 shaped or branching cavities called glands are formed in this 

 tissue, which secrete special fluids, as the saliva, gastric juice, etc. 



The connective tissue, made up of cells separated by various 

 kinds of intercellular substance, is an important part of thebody. 

 In some cases the intercellular substance consists of white inelas- 

 tic fibers, forming a tissue from which the ligaments and tendons 



