2IO ' 1'^TODA. 



In Polystotninn inlc^rrimum, 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 devel- 

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



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

 vided (i) 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 

 existence the larva attaches itself to the gills of a tadpole. The rings of cili- 

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

 commence to be formed on the posterior disc. When the bladder of the tad- 

 pole 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 may be present within 

 the parent. It seems probable that the second and third generations are 

 produced asexually, the generative organs not being developed ; while the 

 young Gyrodactylus of the first generation springs from a fertilized ovum 

 (Wagener, No. 214). 



CESTODA. 



On anatomical grounds the affinity of the Cestoda to the 

 Trematoda has been insisted on by the majority of anatomists. 

 The existence of such intermediate forms as Amphilina tends to 



