Ultrastructural reconstruction of the endodermal nerve net of Hydra vulgaris

The cnidarian Hydra vulgaris has a simple nervous system with as little as a few hundred neurons distributed in two nerve nets in the ectoderm and endoderm. Using this simple neural chassis, Hydra can paradoxically perform relatively sophisticated behaviors, such as somersaulting and feeding. To understand how this simple nervous system is organized, we have performed a partial ultrastructural reconstruction of the endoderm nerve net of a small Hydra specimen. Neurons can be classified into 5 morphological subtypes, most of which connect via specialized interdigitations of their neurite tips, resembling handshakes. Neuronal processes cross the mesoglea, providing a means to coordinate the activity of the ectoderm and endoderm of the animal. Neurons have several different types of vesicles, including clear and dense core ones, which could support synaptic transmission. However, most vesicles are located far from other neurons. Moreover, neurite handshakes are mostly devoid of vesicles. We speculate that Hydra ’s endodermal nerve net operates as a non-synaptic circuit, using a neuroendocrine chemical network to implement its functional operations.