CONTRIBUTIONS TO AMERICAN HELMINTHOLOGY. 71 



fied that Polysfcomum is descended from a Gyrodactylus-like ancestral 

 form. The suckers of Polystomum are not developed simultaneously, 

 and Sphyranura is a transition form, wliere 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 Yan Beneden's family " Gyrodactylida."^^ 



CESTODES. 



Taenia dispab. 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. 



^ Becherehes sur les Trematodes marins. Van Ben. and Hesse, p. 121. 



