194 



ZOOLOGY 



quence the organs referred to are often very much simplified and degenerate. The 

 digestive tract, for example, may be entirely lost. Reproduction by transverse 

 division is not uncommon. By this method strobilae or chains of more or less 

 closely connected individuals occur (Figs. 91, 93). The sexual organs are ex- 

 ceedingly complex, particularly in the parasitic members of the group (Fig. 94). 

 The development is in some instances direct, in others indirect. The principal 

 classes are the Turbellaria, Trematoda and Cestoda. 



Fig. 88. Diagratnof a Turbellarian, showing the generalarrangementof the nervous structures 

 and one of the modes of occurrence of the excretory tubules, which in this case open separately into 

 the pharynx, on the ventral side of the animal. 6, brain; e, eye-spots; ex, excretory canals con- 

 sisting of a transverse portion passing from the mouth toward the dorsal side (see also Fig. 89), 

 and longitudinal tubes which branch into the capillary vessels terminating in /, the flame cells; /.£., 

 lateral nerve cords; m, mouth. 



Questions on the figure. — Compare this figure with the next and identify the 

 structures shown in both. What other positions of the mouth do you discover in 

 the Turbellaria, as figured in reference texts? What other arrangement of the 

 excretory canals and pores? 



Class I. Turbellaria {Planarians, etc.). — These are mostly small non-parasitic 

 Platyhelminthes with a ciliated ectoderm. They are chiefly aquatic and are car- 

 nivorous. The ventral mouth may be anterior, posterior, or median in position. 

 It opens into a muscular eversible pharynx, which may be used to assist in locomo- 

 tion as well as in capturing food. The digestive tract may be simple or very much 

 branched. The brain consists of a pair of ganglia in the anterior region. From 

 the brain lateral nerve cords pass backward through the body. The excretory 

 organs (Figs. 88, 89) usually consist of two or more longitudinal tubes which open 

 on the exterior separately or by a common orifice. The position of the opening 



