56 ESSENTIALS OF ZOOLOGY 



velops various protective structures in the different groups of ani- 

 mals : the hard, horny chitin of insects ; scales of fish ; horny plates 

 and scales of reptiles ; feathers of birds ; hair and nails of mammals. 

 The glands of the body are developed from epithelium. Secretions 

 from these various glands lubricate the surfaces, contain enzymes 

 for digestion of food, supply regulatory substances directly to the 

 blood, serve as poison to other animals, and some are repellent to 

 enemies. 



Sustentative Tissue.— This type comprises all tissues whose func- 

 tion is to bind together or support the various parts of the body. 

 Connective tissue is, in most cases, composed of slender cells with 

 an abundance of intercellular material. This tissue is almost uni- 

 versally present in the various organs throughout the body. Ten- 

 dons, the tough cords that connect muscles to bones, of which the 



ft 



A 



Fig. 14. Germ cells. A, Ovum of female; B, spermatozoa of the male. 



' ' hamstring " is a good example, and much of the dermis of the skin 

 are composed of connective tissue. Bone and cartilage, which make 

 up the framework of the body and support the other tissues, are 

 called supporting tissues. In crayfishes and grasshoppers the sup- 

 porting tissue is chitin instead of bone or cartilage. Cartilage is com- 

 posed of scattered cells interspersed with abundant, homogeneous, 

 granular, semisolid matrix or intercellular substances. Bone is some- 

 what similar, except that the matrix has been replaced by a heavy 

 deposit of calcium phosphate and calcium carbonate, two solid salts. 

 The scattered cells are present as bone cells. 



Muscular or Contractile Tissue. — This is distinctive because of its 

 ability to contract and in that way produce movements. Cells 

 adapted to this function are more or less elongated and fiberlike. 



