Endocrine organs, as explained earlier are those organs or tissues, which release chemical substances directly into blood stream. Not like exocrine glands they do not have ducts to carry their secretion, and are hence considered ductless glands. Table lists the endocrine glands of the higher invertebrate groups, the secretions they generate and the functions of the secretions. Among non-chordates neurosecretory cells (NSCs) form a significant constituent of the endocrine system.
Table: Endocrine Organs
These are nerve cells specialised for hormone production. Thus they combine neural and endocrine properties. Apart from this, endocrine glands may arise from the nervous system, by complete modification. Corpus cardiacum of insects is one such gland. Several non-chordates have also epithelial endocrine glands derived from embryonic ectoderm, mesoderm or endoderm layers. The optic glands of cephalopod molluscs, androgenic glands and Y-organs of crustaceans, and the corpus allatum and prothoracic glands of insects are some instances. You will be studying these structures and their functions in detail. But previous to we proceed further, let us look into the concept of neurosecretion a little more elaborately, since neurosecretion plays a significant role in regulating metabolic and reproductive functions in non- chordates.