UNSEGMENTED WORMS. 



187 



very much simplified and degenerate. The digestive tract, for example, 

 may be entirely lost. Reproduction by transverse division is not uncom- 

 mon. By this method strobilae or chains of more or less closely connected 

 individuals occur. The sexual organs are exceedingly complex, particu- 

 larly in the parasitic members of the group (Fig. 92). The develop- 

 ment is in some instances direct, in others indirect. The principal classes 

 are the Turbellaria, Trematodes and Cestodes. 



FIG. 87. 



FIG. 



FIG. 87. Diagram of transverse section of a Turbellarian through the region of the 

 mouth, d.m., dermo-muscular wall containing longitudinal fibres; ex, excretory system; 

 f, flame cells; g, gut; I.e., lateral nerve cord; tn, mouth; m.f., muscle fibres; ph., 

 pharynx; t, testis; u, uterus; y, yolk glands. 



Questions on the figure. Determine with care the relation of this 

 to the preceding diagram and identify the common structures. What new 

 structures are represented here? What would be their position in the 

 former figure? The great range in position of the muscle fibres and the 

 spongy character of the body contribute to what powers? 



FIG. 88. Diagram of flame cell, the internal terminus of the excretory tubules, c, 

 cilia lining the tubule; f, special cilia constituting the -flame; n, nucleus of flame cell; 

 p, cell processes; v, vacuole or cavity in cell communicating with the capillary 

 tubules ((). 



Questions on the figure. What is the function of the cell itself? 

 Of the flame? 



Class I. Turbellaria (Planarians, etc.}. These are mostly small non- 

 parasitic Platyhelminthes with a ciliated ectoderm. They are chiefly 

 aquatic and are carnivorous. The ventral mouth may be anterior, pos- 

 terior, or median in position. It opens into a muscular eversible pharynx, 

 which may be used to assist in locomotion. 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 



