370 
Te trarliynchus erinaceus 
intestine or mesentery by means of delicate pedicels. In the Gadoid 
larval hosts the cysts are usually situated in the outer wall of the 
stomach, and this is also their site exceptionally in Pleurouectid fishes. 
The cases of the Halibut and Hake already referred to ai’e the only 
instances which I have seen of the infection of the muscles with the 
larvae. In these cases the larvae were enclosed in lenticular cysts 
which lay compressed between the muscle bundles; or they were cylin¬ 
drical worm-shaped bodies usually without scolices and filled with 
calcareous corpuscles and connective tissue, and lying in little tubular 
cavities hollowed among the muscle bundles. In Gurnards and some 
other fishes the cysts are rounded or irregular in shape, less usually 
flask-shaped, and they are nearly always situated on the mesentery and 
hanging freely into the body cavity. The flask-shaped cysts appear to be 
the more typical form, and these are always attached to the mesentery 
by a short stalk which forms the neck of the flask. The size of the 
cyst is very variable, being anything from 1 mm. to about 1 cm. in 
length. 
The cysts are often degenerate so that on dissecting them no trace 
of larval structure can be made out. I have desci’ibed such structures 
as occurring in the Gurnard (1905). In these cases the cysts were 
smaller than usual, round or oval in shape, and pigmented dark-brown 
towards their central portions. On cutting sections they were seen to 
be made up of a number of concentric laminae, composed of some 
substance staining dark blue with Mann’s methyl-blue-eosin, and built 
up of bundles of wavy fibres. In the centre was a nucleus consisting 
of granules, and on the periphery was a layer of tissue continuous with 
that of the host, and containing masses of darkly staining granules. 
The whole structure exhibited a striking resemblance to that of the 
pearls found in the common edible mussel, the appearance presented by 
a meridional section reproducing almost exactly the appearance of a 
decalcified pearl. The origin of these pearl-like cysts in the Gurnard is 
very probably similar to that of the calcareous pearl in Mytilas, that 
is, they are artifacts produced by specific stimulation of the tissues of 
the host by some substance excreted by the parasite. In the mussel 
pearl this substance is excreted by the larva of the Ti'ematode which 
is responsible for pearl production, and since the parasite inhabits the 
shell-secreting mantle lobes, the substance which is secreted round the 
larva is lime laid down in the same way as that forming the nacreous 
layer of the shell. In the case of the pearl-like bodies in the Gurnard 
the substance laid down round the larval Tetrarhynchid is a waxy 
