Class 2. Trematoda. Order 1. Polystomea. 



147 



the host tegin to atrophy, and then wanders (probably through the alimentary 

 canal of the Frog) into the ui-inai-y bladder, where its further development takes 

 place. 



3. Diplozoon paradoxum, "the Double-animal," lives in the gills of different 

 fresh- water Fish. The larva is fui'nished with cilia, which atrophy after it has 

 attached itself to the gills. The young parasite is an elongate animal with two 

 suckers in front and several behind ; there is, further, a median ventral sucker, and 



ABC D 



Pig. 103. Diplomon paradoxum. A free-swimming larvae. B single individual. 

 C two individuals which have begun to unite, the left has seized the dorsal papilla of the 

 light with its ventral sucker. D The same after fusion, each has seized the dorsal papilla 

 of the other. 6 ventral sucker, d gut, h adhesive apparatus of the posterior end, m mouth, 

 r dorsal papilla. — After Zeller. 



iipon the dorsal side, just about opposite to this, a conical papilla. After some 

 time the young animals unite in pairs, in such a way that one seizes the dorsal 

 hump of another with its ventral sucker, and the latter turns ro\md to clasp the 

 dorsal hump of the former in a similar manner, so that the animals are fastened 

 together cross- wise. They coalesce and remain attached tlu-oughout life. After 

 the fusion, the animal stUl gi-ows considerably. 



Order 2. DistomeSB {Bigenetic Trematodes). 



Endoparasites witli one or two suckers, or wholly destitute of 

 them : with a heterogony. The hermaphrodite generation occurs in 

 Vertebrates^ the parthenogenetic in lower animals. 



1. Distomum hepatioum, the Liver-fluke. The hermaphrodite generation 

 frequently occurs in the bUe ducts, especially of Sheep and Cattle (more rarely of 

 other Mammalia) ; length, 3 c/m. Besides the anterior sucker which sun-ounds 

 the mouth, there is a small ventral one at some distance from it. The animal 

 feeds upon blood. The microscopic ova are can-ied by the bUe into the digestive 

 tract of the host, and are conveyed thence to the exterior. If they fall into 

 water or some damp place, each hatches into a ciliated larva pi-ovided with two 

 eyes ; and a small papilla at the front end, by means of which it bores through 

 the skin of a particular species of fresh- water SnaU, Limnseus truncatulus ; if this 

 little Snail is not forthcoming, the larva perishes. In the Snail, the cUiated 

 covering is lost, and niunerous ova are formed ia the small accelous animal, the 

 sporocyst (first asexual generation), during its growth. The ova 

 hatch within the sporocyst, and each develops into a small animal, a redia 

 (second asexual generation), difEeriag from the parent, in the posssssion 



L 2 



