540 



COLLEGE ZOOLOGY 



Protozoa all produce secretions, for ex- 

 ample, the digestive enzymes that are 

 poured into the food vacuoles, and the skele- 

 tons of the foraminiferans and radiolarians. 

 In Metazoa, cells or organs are specialized 

 for secretion of particular substances; such 

 cells or organs are called glands. Unicellular 

 glands occur abundantly in the lower in- 

 vertebrates, such as those that secrete the 

 spicules and spongin in sponges, and the 

 hypnotoxin in the nematocysts of coelen- 

 terates. They also occur in higher animals, 



including vertebrates, where they may be 

 found in all mucous membranes. Most 

 glands in higher animals, however, are mul- 

 ticellular and consist of epithelial tissue spe- 

 cialized for this purpose. Some of the types 

 of glands are illustrated in Fig. 391. 



Glands and their secretions may be sepa- 

 rated into two types: (1) glands that secrete 

 directly to the outside or into a cavity, or 

 discharge their products through a duct, are 

 duct (exocrine) glands and produce exter- 

 nal secretions; and (2) glands that discharge 



Non-glandular 

 epithelium 



Glandular 



epithelium 



Simple alveolar 



Branched tubular 



Coiled tubular 



Branched alveolar 



Much-branched alveolar 



Figure 391. Diagram showing some types of glands. 



their products directly into the blood or 

 lymph are ductless or endocrine glands and 

 produce internal secretions. Glands of ex- 

 ternal secretion vary greatly in the character 

 of their products. Some of the principal 

 types are as follows. 



1. Protective glands that secrete mucus, 

 as in the skin of the earthworm and frog; 

 exoskeletons, as the cuticle of the earth- 



worm, crayfish, and grasshopper; and shells, 

 such as those of mussels. Many types of 

 glands secrete substances that protect the 

 eggs of animals: the cocoon of the earth- 

 worm, the egg case of the cockroach, the 

 shell of birds' eggs, and the substance that 

 fastens the eggs of the crayfish to her swim- 

 merets. 



2. Skeletal glands that secrete the calcare 



