Phenomena related to neoplastic growth have been studied in a number of invertebrate phyla, and tumors of either epithelial or connective tissue origin have been reported in annelids, sipunculids, arthropods, molluscs, and ascidians. Evidence of the true neoplastic character of such alleged tumors is often scanty; reasonably convincing signs of malignancy are found only in a small number of reported cases.
A number of tumors described in representatives from various invertebrate groups may be classified as spontaneous growths, since their cause is unknown. In insects certain of the alleged tumors are known to be hereditary. Other factors reported to cause tumors or abnormal growths in invertebrates are parasites, endocrine imbalance, and disturbance of innervation. Carcinogens have been tried on a variety of invertebrates and have been reported to cause tumors only in molluscs. X-rays in suitable dosage are known to cause tumor-like growths or increase their incidence in insects (Drosophila).