This study aimed to investigate the histological and histochemical
characterizations of the ink gland of the sea hare “Aplysia argus”.This gland
is composed of two covering epithelium and an inner matrix. The covering
epithelium involved the cubodial facing the mantle cavity and the columnar
facing the mantle shelf. Underneath the covering cuboidal epithelium, there
are bundles of collagenous fibers permeated by two types of granulated
cells. The columnar covering epithelium underlies an adipose connective
tissue that includes dispersed cells between adipocytes. The inner matrix of
the ink gland possesses three types of secretory vesicles; namely, dark-Red,
granulated and transparent. Histochemical investigation indicated that the
granulated cells content included neutral mucopolysaccharides and protein,
while they were devoid of acid mucopolysaccharides. The dark-red vesicles
possess a mixture of acid and neutral mucopolysaccharides; whereas, the
granulated vesicles contain acid mucopolysaccharides only. The dark-red
and granulated vesicles contain protein contents. The histological
characteristics of the ink gland of Aplysia argus corroborated the general
organization of aplysiids. To the best of our knowledge, new data were
added in the current work (for the first time) about the two different types of
granulated cells and adipose connective tissue, in addition to the
investigation of the histochemical contents of granulated cells, secretory
vesicles and dispersed cells.