On Two Species of the Trematode Genus Didymozoon from the Mackerel

Abstract

Taschenberg (1879) briefly described, under the name of Didymozoon scombri, a trematode which he recorded as occurring in pairs in cysts on the inside of the gill-covers of Scomber colias at Naples. Odhner (1907) found what he believed to be the same species not uncommonly in small specimens of S. scombrus off the west coast of Sweden, but was unable to find it in the larger fish taken in the early summer. He gives a minute description and excellent figures of the worm. Johnstone (1914) has also given a good, though diagrammatic, figure of its anatomy. Odhner describes the worm as occurring most commonly in paired cysts on the upper pharyngeal bones, but also in exceptional cases in cysts on the inside of the gill-covers and on the outside of the head, close behind the eye. About half the cysts examined by him contained two worms, the rest from three to seven. Johnstone describes the cysts as being most commonly situated ” “on the roof of the mouth beneath the pharyngo-branchials”, but also sometimes on the basi-branchials, on the external surfaces of other parts of the gill-bars, or on the internal surface of the operculum. One cyst contained as many as sixteen worms. According to Dollfus (1926), he and Monod found cysts containing one, two, three or more worms in the mucosa of the mouth, palate and gills of S. scombrus, and also single unencysted worms under the transparent skin behind the eye.Thus at least two authors record the same worm as occuring both inside the mouth and on th e outside of the head, behind the eye.