iq6 zoology 



varies very much in the different orders. The tubules are much branched 

 interiorly and penetrate the soft tissues of the body as minute capillaries with 

 thin walls. They terminate in cells of special structure which are excretory in 

 function. A group of cilia (the flame, Fig. 90, f) helps in creating a current in the 

 capillary tubes. The lining of the tube may also be supplied with cilia. Repro- 

 duction is by division {fission, Fig. 91) or by eggs and sperm. The Turbellaria 

 have remarkable powers of regenerating lost portions. Experiments show that 

 very small portions of an individual will, under favorable conditions, reproduce all 

 the parts of a complete animal. In habit they may be terrestrial, fresh-water or 

 marine. They vary in size from microscopic fresh-water forms to a length of six 

 inches or more in the case of the marine and land types (Figs. 88-91). 



Class II. Trematoda. — The Trematodes are small, usually parasitic, Platyhel- 

 minthes. The ectoderm is provided with a protective "cuticle" and is conse- 

 quently destitute of cilia. They possess a well-developed and often much- 

 branched digestive sac, which has only one opening — the mouth. Usually one or 

 more sucking discs are present. By means of these the parasite attaches itself to 

 the host. The nervous and excretory systems are similar in general to those of the 

 Turbellaria, but are somewhat better developed and more complex. In those 

 members of the class which are external parasites there is usually no metamorphosis 

 in the development. In the internal parasites, as the Liver-fluke of the Sheep, 

 there is a complicated metamorphosis. A Liver-fluke {Fasciola hepatica) is found 

 in the bile ducts of the liver of the sheep, where it gives rise to a much-dreaded 

 disease — "liver rot." The eggs which are formed, fertilized and pass through 

 the early stages of cleavage here, pass out of the bile ducts to the intestine and 

 thence to the exterior. If the larva reaches water it develops into a free-swim- 

 ming larva (Fig. 92, C), whicli to insure further development must bore into the 

 tissues of a particular pond-snail (Limncza truncatula). It there develops into a. 

 kind of sac (sporocyst) in which a number of parthenogenetic ova are produced. 

 These ova develop into rediiB (Fig. 92, F) which escape from the sporocyst. 

 These redias, in turn, produce parthenogenetic ova which may develop into other 

 redise or a very different, tailed form known as a cercaria (Fig. 92, G). These 

 cercarise escape from the snail into the water, ultimately encyst on damp grass, 

 and are swallowed by sheep. The larvas find their way to the liver and develop 

 there again into the adult fluke. It is evident that such a form must have im- 

 mense powers of reproduction, when it is considered that the reproduction takes 

 place at several points in the life cycle (Fig. 92, -(- *). This compensates for the 

 great loss of life involved in changing from host to host. It is said that a single 

 fluke may produce half a million eggs. The disease is prevalent only in those coun- 

 tries ^here this species of LimniBa occurs. It is much worse in wet years. Millions 

 of sheep have died in England alone, in a single year, from the attacks of this para- 

 site. Trematode parasites are common ariiong animals and frequent most diverse 

 organs. As compared with the Tttrbellaria, the Trematodes have lost their eye- 

 spots, have less well developed sens^ organs and central nervous systems and have 

 highly elaborate reproductive organs and metamorphosis. These facts are related 

 to the parasitic habit. 



