Morphological and histochemical observations on the nervous system of two sipunculid species, Siphonosoma australe and S. cumanense

Abstract

The nervous systems of Siphonosoma australe Keferstein and S. cumanense Keferstein, 1867 are histologically similar. However, the two species differ with regard to the structure of the nuchal organ and ventral sensory organ. The nuchal organ in S. cumanense is multilobulated, innervated by 4 pairs of nerves and secretes acidophilic granules, whereas that in S. australe is bilobed, innervated by 2 pairs of nerves and is nonsecretory. Whilst the ventral sensory organ in S. cumanense appears empty and collapsed, that in S. australe contains concretions rich in glycoprotein. The central nervous system consists of the cerebral ganglion, circumesophageal commissures, and the ventral medullary nerve cord. The ovoid brain consists of a centrally placed neuropile and a cellular cortex. Three types of neurons are found in the brain: the giant (cyanophil), median (acidophil), and the globular neurons. The cell bodies of the giant and median neurons are located in the posterodorsal part of the brain, whereas the globular neurons predominate in the rest of the cortex. Most of the protein tests stained the giant and median but not globular neurons. The giant neurons also showed a weak PAS and sudanophilic reaction, suggesting the presence of small amounts of neutral mucopolysaccharides and lipids. Cyanophils and acidophils containing fuchsinophilic granules show possible neurosecretory activity.