Although all living cells generally secrete some or the other substances, some cells, such as the gobbler cells of ciliated and certain other epithelia, are highly specialized only for secretion it resembles columnar epithelium but its cells are larger in size and from different types of chemical substances. These cells from glands in all vertebrates, which are mainly of two types:

1) Unicellular Glands:

These glands are formed of only one cell. They ae found in the mucous layer of small and large intestines. They ae flask shaped, therefore, they are also known as goblet cells.

2) Multicellular Glands:

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They are formed of many glandular cells and are situated deep in the body. They are of two types.

3) Alveolar glands:

They are of two kinds – 1) Simple Alveolar, and (2) Compound Alveolar:

Simple Alveolar bag like and open to the outside by a small pore. Their inner portion is circular. They are found in the skin of frog and open by fine ducts. In the epidermis mucous and poisonous substances are secreted by them which come out on the surface of the skin, keeping.