i88 



ZOOLOGY. 



through the body. The excretory organs (Figs. 86, 87) usually consist of 

 two or more longitudinal tubes which, open on the exterior separately or 

 by a common orifice. The position of the opening varies very much in 

 the different orders. The tubules are much branched interiorly and pene- 

 trate the soft tissues of the body as minute capillaries with thin walls. 

 They terminate in cells of special structure which are excretory in func- 

 tion. A group of cilia (the flame, Fig. 88, /) helps in creating a current 

 in the capillary tubes. The lining of the tube may also be supplied with 

 cilia. The Turbellaria have remarkable powers of regenerating lost por- 

 tions. 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. 86-89). 



FIG. 89. Diagrammatic sagittal section of Microstomum, showing a chain of four 

 zooids produced by fission, b, brain of the original zooid (the exponents indicating 

 corresponding structures of the more recently formed zooids) ; c,. ciliated pit; d, dis- 

 sepiments indicating different stages in the separation of the zooids; e, eyespot; ent, 

 entoderm; g, gut; gl., glandular cells about the mouth; m, mouth of the original worm. 



Questions on the figure. What various evidences can be found of 

 the relative age of the zooids? Is the mouth formed apparently from 

 entoderm or ectoderm? Is the gut a blind sac? What incidents seem 

 necessary when this chain separates at the oldest plane of division, and 

 forms two chains, in order that each may be like the parent? How is 

 this like segmentation in annulates (see Fig. 99)? How unlike? 



Class 2. Trematoda. The Trematodes are small, usually parasitic, 

 Platyhelminthes. The ectoderm is provided with a protective " cuticle " 

 and is consequently 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 some- 

 what 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, 



