THE OLFACTORY APPARATUS OF PETROMYZON MARINUS L.

Abstract

The olfactory organ of Petromyzon marinus was studied anatomically and histologically in adult and larval forms and compared with that of related cyclostomes as reported in the literature. The single olfactory organ of this species consists of three parts: the nasal tube, the olfactory sac, and a nasopharyngeal pouch. The tube is lined with indifferent epithelium and contains a valve at the entrance of the sac. The valve directs a jet stream of water into the sac when water penetrates the nasal tube. The structure of the valve allows it to reverse its position when water is expelled from the organ. The sac or nasal chamber is divided by 25 folds of its walls, 13 on the right, 12 on the left side. One of the folds, in median dorsoventral position, divides the sac into two chambers but for the anterior extremity of the sac. The folds are lined with epithelium which contains the olfactory cells. The latter are not distributed evenly in the epithelium but are limited to those areas of the folds which face a neighboring fold. The peaks and valleys of the folds are lined with indifferent epithelium. The connective tissue in the posterior and dorsoposterior regions of the sac contains clusters of cells within blood sinuses. These clusters are the accessory olfactory organ; no open communication could be found between the clusters of cells forming this organ and any other part of the olfactory organ. The organ is innervated by rami of the n. olfactorius which is formed by the sensory cells of the folds in the nasal sac. The whole is surrounded by the cartilaginous nasal capsule. The pouch is a continuation of the nasal tube in ventral and ventrocaudal direction and lies between the digestive tract and the nasal sac and brain. Its posterior part is enlarged laterally and ends as a blind pouch at the level of the second internal gill opening. Since the pouch is not protected by rigid cartilage it is subject to the rhythmic contractions and relaxations of the respiratory muscles and body walls, thus sucking in water during the inspiration and expelling it during the expiration phase.