A survey of neuroendocrine phenomena in non-arthropod invertebrates.

Abstract

SUMMARY Neurosecretory phenomena are apparently ubiquitous among Metazoa. In Hydra, neurosecretory products are probably involved in the control of growth and reproduction. Secretory elements in the central nervous system of turbellarians probably promote fission, regeneration and reproduction. The cerebral ganglia of nemertines are the source of a hormone which exercises an inhibitory influence on maturation of the gonads and the development of somatic sexual characteristics. A principle secreted by the ganglia and/or the associated cerebral organs controls weight regulation. A sex hormone controls sexual differentiation. Secretory neurons may influence the production of exsheathing fluid and ecdysis in nematodes. In nereid polychaetes, a single hormone which originates from the cerebral neuroendocrine system apparently promotes segment proliferation and inhibits maturation. However, in certain other polychaetes, contrasting endocrine mechanisms seem to operate. In lumbricid oligochaetes, a hormone secreted by the cerebral ganglion (and possibly by other nervous centres also) promotes gonadal activity and the differentiation of somatic sexual characters. In some species the ganglion exerts an inhibitory influence on the associated processes of ‘diapause’ and posterior regeneration. The cerebral and suboesophageal ganglia are implicated in the control of osmoregulation. Cerebral neurosecretory cells in limicolous oligochaetes may influence regenerative growth and osmoregulation. The cerebral ganglia of leeches secrete a gonadotrophic hormone. In gastropod molluscs, hormones secreted by the cerebral ganglia, glandular ‘dorsal bodies’ and/or optic tentacles are responsible for the control of the development of the reproductive tract, and the activity of the gonads. Contrasting mechanisms are thought to operate in the different gastropod groups and there is conflicting evidence particularly with respect to the endocrine functions of the optic tentacles and the gonads. The ‘bag cells’ of the abdominal ganglion of ApZysiu secrete a hormone which induces egg‐laying. Cells in the pleural and parietal ganglia probably control osmoregulation in Lymnaea. The optic glands of cephalopods secrete a gonadotrophic hormone, but sex hormones are apparently absent. The functional significance of the neurovenous tissues (presumptive neurosecretory complexes of unusual character) is obscure in most cases. The radial nerves of starfish are the source of a hormone which induces the production of I‐methyl adenine by the follicle cells of the gonad. This second principle stimulates oocyte maturation and the shedding of male and female gametes. The relevance of such information to the development of certain biological concepts and to various aspects of comparative physiology is briefly discussed.