UNSEGMENTED WORMS 1 95 



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, /) 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 most complicated metamorphosis coupled with an alternation of sexual 

 and non-sexual generation (see 223). A Liver-fluke (Distomum hepaticum) 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- 

 swimming larva (Fig. 92, C), which to insure further development must bore into 

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

 a kind of sac (sporocysf) from the inner cells of which special cells are budded 

 (Fig. 92, ). These cells have the power of developing into embryos of a second 

 generation by cell division that is to say, non-sexually. Several such non-sexual 

 reproductions may occur in the body of the snail (Fig. 92, +). These later genera- 

 tions of larvae pass, often by the death of the snail, into the water, whence they may 

 enter the alimentary tract of the sheep in drinking. The larvae find their way to 

 the liver and develop there again into the adult fluke. It is evident that such a 

 form must have immense powers of reproduction, when it is considered that the 

 reproduction takes place at several points in the life cycle (Fig. 90, + *). This 

 may be seen to be a necessity to compensate 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. Each of these which succeeds in reaching the host snail may produce 

 thousands of the last generation of asexual individuals, and yet the numbers of the 

 species probably do not increase. The disease is prevalent only in those countries 

 where this species of Limncza 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 parasite. 

 Trematode parasites are common among animals and frequent most diverse organs. 

 As compared with the Turbellaria, the Trematodes have lost their eye-spots, have 

 less well developed sense organs and central nervous systems, and have highly 

 elaborate reproductive organs and metamorphosis. These facts are related to the 

 parasitic habit. 



