J. Johnstone 
367 
Fixation for minute histological study is best effected by the use of 
Zenker’s fluid ; other mercuric chloride fixatives do not appear to give 
nearly such good results. Fixation in vom Rath’s fluid was also tried, 
followed by staining in iron haeiliatoxylin but the stain is not a 
general one, although good for some details. 
The sexually mature worm is comparatively rare in Irish Sea 
fishes, and as a general rule I have had to dissect from ten to twenty 
fishes in order to obtain one tapewoi’m. This paucity of material has 
rendered the investigation, so far, an incomplete one. Obviously the 
details of the life-history, such as the development of the Oncosphere 
and Plerocercoid stages, and the precise means of infection of the larval 
and final hosts, can only be properly studied by feeding experiments 
and for this purpose abundant material has not yet been available. 
Lack of material has also prevented some much needed work on the 
finer details of the histology of the central nervous system, since in 
investigations of this nature many experiments have to be made before 
satisfactory methods of fixation and staining can be evolved. 
The Plerocercoid larvae of Tetrarhynchids, chiefly T. erinaceus, are 
among the most abundant of Helminth parasites of some Irish Sea 
fishes. I have observed them from Trigla gurnardus and T. hirundo, 
Gadus callarias, 0. riierlangus, G. aeglefinus, Merluccius vulgaris, 
Scomber scomber, Hippoglossus vulgaris, Pleuronectes platessa, and 
Arnoglossus laterna, and there are no doubt other larval hosts. The 
cysts containing the larvae are generally abundant, and it is rare, in my 
experience, for Whiting and Gurnards to be free from infection. In 
a Whiting stomach in the Fisheries Laboratory at Piel, Barrow-in- 
Furness, there are over 130 encysted larvae in the wall; and in a 
Halibut stomach examined some time ago there were nearly as many. 
The larvae may be found in various parts of the tissues of the host but 
they are very generally found on the mesenteries, or are present on the 
peritoneum covering the stomach and intestine : I have never seen 
them in or on the liver*, or free in the body cavity. They are present 
exceptionally in the muscles: this was the case in several Ashes—Hake 
and Halibut—examined by me. In these Ashes the flesh round the 
vertebral column contained numbers of oval or cylindrical cysts which 
included Plerocercoid larvae. Little cavities had formed among the 
muscle bundles by the intrusive organisms. Many of these larvae were 
' Since writing the above I have obtained four specimens of the peculiar Tetrarhynchid 
Coenomorphus linguatula (van Beueden) from the Cod-Fish (Gadus vireus). It was 
attached to the liver of the host. 
