The principal ganglia of the central nervous system (CNS), of the sea hare, Aplysia oculifera, are concentrated around the oesophagus, caudally from the buccal mass. The usual complement of such ganglia comprises paired cerebral, buccal, pleural and pedal ganglia. The neurosecretory cells (NSCs) were variably observed in all the above mentioned ganglia. They were differentiated according to size and shape into three cell types namely: large (type A), medium (type B) and small (type C) cells. The A cell type is less common and has a large size with an oval or flattened shape. It measures from 116.6 x 69.9 \i to 34.8 x 19.5 |i, with a centrally located irregularly shaped or polymorphic nucleus, with many small intensely colored nucleoli. The B cell type is more numerous and has an oval or rounded shape, with a rounded and centrally located nucleus, which has many small intensely colored nucleoli. It measures from 59.5 x 40.8 [x to 22.2 x 18.1 ji. Meanwhile, C cell type constitutes the majority of the NSCs. It is small rounded or ovoid cell, measuring from 35.6 x 31.5 \x to 5.6 x 5.0 \x, with a large rounded nucleus, having a sharply defined eccentrically placed nucleolus.
The ultrastructure of NSCs of these ganglia was also investigated. Generally, it showed active Golgi body, rough endoplasmic reticulum and a great number of free ribosomes. The electron dense neurosecretory granules were observed. These were attributed to the activity of both Golgi apparatus and rER that have the main role in their formation.