The chemical integrating system in animals that lack a vertebral (spinal) column. An endocrine system consists of those glandular cells, tissues, and organs whose products (hormones) supplement the rapid, short-term coordinating functions of the nervous system. Evidence has been presented for hormones in a wide variety of invertebrates. However, the large majority of the published reports pertains to the more highly evolved groups that will be discussed below, the annelids, mollusks, and particularly two classes of arthropods, the insects and crustaceans. Many of the hormones in invertebrates are neurohormones; that is, they are produced by nerve cells. Just as in the vertebrates, a wide variety of functions are regulated by hormones in the invertebrates.