worm;^. 



195 



Fig. 176. — Jjip- 

 orpa. 



Fig. 177. — Egg of Diplozom. 



by Zeller. The larva was formerly supposed to be a distinct creature, and went by the 

 name of Diporpa, Pig. 176. The Biplozoon is the mature sexual form, which produces 

 the eggs, long oval capsules, with a long snarled thread running off from 

 one end. The egg. Fig. 177, breaks open, and the larva swims about in 

 search of its host, to which, when found, it attaches itself upon the gills, 

 living there, in company with the adult, perhaps 

 for months, but after a while they pair off ; there 

 is a little knob on the back of each Diporpa, and, 

 of course, a ventral sucker ; when two join they 

 twist over so that each seizes with its sucker the 

 dorsal knob of the other, and so they remain, 

 and in due time actually grow together. They 

 are, in truth, the most monogamous of animals, 

 for each individual can have one mate only, from 

 whom he can never be divorced. The union takes 

 place in such wise that the animals form a cross. 

 The left tail belongs to the right head. Each member of the 

 Diplozoon has nine suckers, two in front, by the mouth, one 

 near the middle, and six at the posterior extremity of the body. 

 Dr. Ernst Zeller has worked out very carefully the complicated history of a typical 

 species of the second group of trematods, namely, the Polystomum integerrimum, 

 parasitic in the bladder of frogs. The animal grows to a third of an inch in length, 

 and is remarkable for having, unlike most Trematoda, a branching intestine; the 

 posterior end of its body is expanded into a broad disc, with three pairs of suck- 

 ers on its under side. The eggs are discharged by the parent in the bladder, and ex- 

 pelled into the water. The larva hatches out in from fourteen to forty days, accord- 

 ing to the temperature. " The young worm," writes Dr. Zeller, " is an extremely 

 lively, active animal, and swims about merrily in the water by means of its coat of 

 cilia ; contracting and stretching its body, bending and turning, and often, also, bend- 

 ing its head down, turns a somersault as quick as a flash." Under ordinary conditions 

 the eggs are laid in the spring, when the frogs awake from hibernation, and the larvae 

 are hatched at a period when the tadpoles are in a somewhat advanced stage of evolu- 

 tion. From the water the active larvje get into the branchial chambers of 

 the tadpoles, where they take their abode for about two months ; when 

 the gills of the frog begin to disappear they migrate through the oeso- 

 phagus and intestine to the bladder, and in three years attain sexual 

 maturity. On the other hand, when the formation of the eggs and the 

 evolution of the Polystomum larvae are artificially accelerated by keep- 

 ing the frogs in heated rooms, the larvae are hatched at a period when 

 the tadpoles are quite young and their gills very delicate. Their evolu- 

 tion is then very rapid. They become mature and produce eggs within 

 five weeks ; their life is at an end before the gills of their hosts are 

 obliterated, and they never migrate into its internal organs. The 

 remarkable conclusion of the varying life-history is a difference in the 

 adult, for the gill-cavity Polystoma are very unlike the normal adults 

 in form, appearance, and their whole anatomy. External circumstances 

 here produce a maximum effect, for when they are changed in a certain manner the 

 same eo-ws which would nominally produce the ordinary Polystomum integerrimum. 



Fig. 178.— Young 

 of Polystcymwrn. 



