CONTRIBUTIONS TO AMERICAN HELMINTHOLOGY. 71 
fied that Polystomum is descended from a Gyrodactylus-like ancestral 
form. The suckers of Polystomum are not developed simultaneously, 
and Sphyranura is a transition form, where the formation of these is 
restricted to one pair. 
The consideration of the probable relationships of the hosts of 
these forms lends additional authority to such a conclusion. If the 
piscine ancestors of Amphibia had Gyrodactylus-like gill-parasites, 
these would probably be transmitted to their descendants, and we 
should not be surprised that among the oldest representatives of 
these, two (the Frog-larva and Necturus) should possess such. The 
texture of the gills in Necturus might account for the change in the 
caudal armature. The loss of the gills in the Frog is necessarily 
accompanied by a change of habitaculum on the part of the parasite ; 
and it is not surprising that the emigrating worms should have pros- 
pered so well in a locality where so many favourable conditions 
obtain as in the urinary bladder of the same host. That some 
Chelonia are the only reptiles in which parasites belonging to the 
same series have been found is probably to be accounted for by their 
aquatic habits. 
Dactylogyrus may be regarded as a divergent form marked by its 
peculiar genital armature, the shape of the eggs, and the arrange- 
ment of the caudal hooks. In all of these points the three other 
genera approach each other more closely, and as Gyrodactylus is 
- evidently nearer the stem-form than the others, all might be received 
into Van Beneden’s family ‘“ Gyrodactylida.”* 
CESTODES. 
TAENIA DISPAR. Goeze. 
I have to record another habitaculum for this worm. The speci- 
men of Rana halecina above referred to (p. 6), expelled several ripe 
proglottides which seem to be much smaller than usual, as will be 
seen from the measurements given below. In the intestine of the 
frog were found several chains about an inch and a half in length, 
and also many scolices and immature chains of different lengths. 
Many more worms in the two latter conditions were also found in 
the body cavity between the viscera; whether these become mature 
in this position I am unable to say—they certainly frequently occur 
here. 
21 Recherches sur les Trematodes marins, Van Ben, and Hesse, p. 121. 
