WORMS. 



195 



Fig. 177. — Egg of Dipioz, 



.by Zfller. Tlic larva was formei-Iy sujiposed to be a distinct creature, and went by the 

 name of Diporpa, Fig. 176. The Diplosoon is the mature sexual form, which produces 

 tlie 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 Dij)or])a, 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. Tlu y 

 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 ^'^- '^'^- " ^'P' 



orpa. 



])lace in such wise that the animals form a cross. 

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

 T>lplozoo)i 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 Polystonnmi inte(/errimmn., 

 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 dischai'ged 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 temperatuie. "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." Tender ordinary conditions 

 the eggs are laid in the spring, wlien the frogs awake from hibernation, and the lar\:ie 

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

 tion. From the water the active larva? get into the liranchial 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- 

 piiagus and intestine to the bladder, and in three j'ears attain sexual 

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

 evolution of the Folystomam larva? are artificially accelerated by keep- 

 ing the frogs in heated rooms, the larvas are hatched at a jicriod when 

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

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

 rive 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 Pohjstoma are very unlike the normal adults 

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

 here produce a maximum effei-t, for when they are changed in a certain manner the 

 same eggs which would lUfniinally produce the ordinary Pohjstorn^trn integerrinaan 



Via. 178.— Young 

 of J'olt/siomum. 



