PL A T YKL MI NT HEX. \ 7 3 



to Cercarise or fresh Redia?) with true sexual reproduction there is 

 in addition a true alternation of generations. 



Polystomeae. The ova of the Polystomea are usually large and 

 not very numerous, and they are in most cases provided with some 

 process for attachment. Some species of Polystomese, e.g. Gyrodac- 

 tylus, are however viviparous. The young leave the egg in a nearly 

 perfect state, and at the utmost undergo a slight metamorphosis and 

 no alternations of generations. Some however (Polystomum, Diplo- 

 zoon) are provided with temporary cilia, but the number investigated 

 is too small to determine whether ciliation is the rule or the excep- 

 tion. The ciliated larvae have a short free existence. The cilia are 

 developed on special cells which may be arranged in transverse bauds 

 in the same way as in the larva) of many Chsetopods, but are not, 

 in the larva? at present known, distributed uniformly. When the free 

 larvce become parasitic the cells with cilia shrink up. 



In Polystomum integerrimum, which lives in the urinary bladder of Rana 

 temporaria, the eggs when laid in the spring pass out into the water. The 

 segmentation is complete, and the embryo when hatched is provided with 

 most of the adult organs, but presents certain striking larval characters. 

 It has five rings of ciliated cells. Three of these are placed anteriorly, and 

 are especially developed on the ventral surface, the posterior one being 

 incomplete dorsally ; two are placed posteriorly, and are especially de- 

 veloped on the dorsal surface. Anteriorly there is a tuft of cilia. 



The larva itself resembles somewhat an adult Gyrodactylus, and is pro- 

 vided (1) with a large posterior disc armed with hooks, and (2) with two 

 pairs of eyes which persist in the adult state. After a certain period of free ex- 

 istence the larva attaches itself to the gills of a tadpole. The rings of ciliated 

 cells shrink up, and some of the six pairs of suckers found in the adult com- 

 mence to be formed on the posterior disc. When the bladder of the tadpole 

 is developed, the young Polystomum passes down the alimentary tract to 

 the cloaca, and thence to the urinary bladder, where it slowly attains to sexual 

 maturity. When the larva becomes attached to the gills of a very young 

 tadpole, its development is somewhat more rapid in consequence of better 

 nutrition from the more delicate gills. It then reaches its full development 

 in the gill cavity, and, though smaller and provided with differently 

 organised generative organs to the normal form, produces generative pro- 

 ducts and dies without being transported to the bladder (vide Zeller, 

 Nos. 216 and 217). 



The ova of Diplozoon., a form parasitic on the gills of freshwater fish 

 (Phoxinus, etc.), are provided with a long spiral filament (Zeller, no. 215). 

 The embryo has five ciliated areas, four lateral and one posterior. The 

 young form is known as Diporpa. Sexual maturity is not attained till two 

 individuals unite permanently together. They unite by the ventral sucker 

 of each of them becoming attached to the dorsal papilla of the other. Sub- 

 sequently these parts coalesce, and the ventral suckers disappear in the 

 process. Gyrodactylus, parasitic, like Diplozoon, on the gills of freshwater 

 fishes (Gasterosteus, etc.), is remarkable for its mode of reproduction. It is 

 viviparous, producing a single young one at a time, and, what is still more 

 remarkable, the young while still within its parent produces a young one, 

 and this again a young one, so that three generations maybe present within 



