152 INVEBTEBBATE MOBPHOLOOT. 



the snail, which thus become enormously distended. The club-shaped 

 structures so formed are abundantly supplied with muscle-fibres, and by 

 rigorous movements finally burst the distended wall of the tentacle, and 

 separating from the Sporocyst fall to the ground. There they move 

 about, resembling an insect larva in general appearance, a resemblance 

 increased by banded markings of green and white, which render them very 

 conspicuous, and they are apt finally to be snapped up by some bird, in 

 whose digestive tract the young Distomes are set free and become mature. 

 There can be little question but that the simple metamorphosis of the 

 PolystomecB represents the original method of development of the Trematoda, 

 the heterogony characteristic of most Distomew being a secondary acquisi- 

 tion developed in accordance with their endoparasitism. An idea of the 

 mode in which this alternation of generations has been brought about is 

 furnished by such forms as Cfyrodacti/lus, in which the development of the 

 ovum takes place within the body of the parent, the young in their turn 

 developing embryos before being born (see p. 147). This acceleration of 

 sexual maturity, accompanied by parthenogenesis, has brought about the 

 condition seen in the Sporocyst or Eedia, which are embryos provided with 

 ova capable of parthenogenetic development. Thus fundamentally the 

 heterogony is a peedogenesis (see p. 60), and may be compared, in a gen- 

 eral way, with the formation of a hydroid colony by the budding of a 

 medusa larva. 



III. Class Cestoda. 



Like the Trematoda the members of this class are para- 

 sites, but are throughout endoparasites, and present a much 

 greater degeneration of structure than is found in the Disto- 

 mese, accompanied by peculiarities of development differing 

 somewhat from what occurs in these forms. The Cestodes 

 or Tapeworms lack all trace of a digestive tract and of a 

 mouth, living in their mature state attached to the wall of the 

 digestive tract of their host, and immersed in the nutritive 

 fluids contained in the intestine. 



In some lorms, such as Caryophyllceus (Fig. 81, ^4), para- 

 sitic in the intestine of Cyprinoid fishes, the similarity to a 

 Trematode is very striking, except in the absence of suckers 

 for adhesion and of a digestive tract. The worm consists of a 

 somewhat dilated head, succeeded by a narrower portion which 

 may be termed a neck and gradually enlarges to the rather 

 cylindrical body, which contains a single set of reproductive 

 organs. In Ligula, which is found in the intestine of aquatic 

 birds, there is likewise au absence of suckers, but the repro- 



