SOME PARASITES OF SAGITTA BIPUNCTATA. 203 
Sagitta, which looks as if Derogenes varicus had a different intermediate 
host in the open sea than it does near the shore. 
The larger larvee of Derogenes varicus which are found in Sagitta have 
nearly all the adult characters (Plate I, Fig. 2), and the smaller ones are 
found in intermediate stages and are easily recognised. That we have 
to do with the true Derogenes varicus is placed beyond a doubt by the 
occurrence of a mature specimen in Sagitta which bears eggs (Plate I, 
Fig. 4). A parallel case is found in Echiurus pallasii (Greef, 1879) which 
contained a mature Distomum, D. echiuri Greef, and other cases of 
trematode larve producing eggs have been recorded, although they are 
rare. The present specimen has only a few eggs, whereas in the ordinary 
adult stage in a fish they are very numerous. 
A curious fact noticed is that all these larval Derogenes varicus are 
beset with small spines, whereas it is a characteristic of the adult that 
although it has sometimes a wrinkling of the skin it is unarmed and 
usually smooth. It is possible that these wrinkles may be the remains 
of the spines fused together. The spines are specially distinct in the 
younger specimens. 
These larval Derogenes varicus are nearly always found in the region 
of the ovary of Sagitta, and there is rarely more than one present in each 
individual, although one may be present at the same time as the larval 
Ascaris described above. 
Pharyngora bacillaris (Molin), the second larval trematode found in 
Sagilta, is a common parasite of the mackerel in its adult state, and has 
been found in the whiting and also a few other fish, except in the whiting, 
in an immature state. Nicoll (1914) found many thousand of the im- 
mature form in Cyclopterus lumpus. These had probably got in with 
the food and would not come to maturity. The late cercaria stage of 
this worm was found frequently in medusee (Lebour, 1916) and free in 
the plankton (Nicoll, 1910). It was also found in ctenophores, so it is 
evidently not particular as to its intermediate host. Cercarie of all 
ages were found in Sagitta occupying usually the region of the ovary, 
as is the case with Derogenes varicus, but sometimes it 1s inside the 
alimentary canal, which looks as if Sagitta swallows it and afterwards it 
migrates through the intestinal wall into the region of the ovary. What 
I have no doubt is the free-swimming cercaria of this trematode was 
found once in tow nettings on January 28th, 1916. Sagitta from the 
same samples contained these cercarie without their tails, and it could 
be traced up to the ordinary Pharyngora late cercaria stage, such as was 
found in the medusze and free in the plankton. The free-swimming 
cercaria is extremely interesting (Fig. 6). It is provided with a large 
tail several times the length of the body and armed with bunches of 
