194 



ZOOLOGY 



FIG. 89. 



FIG. 90. 



FIG. 89. Diagram of transverse section of a Turbellarian through the region of the mouth. 

 d.m., dermo-muscular wall containing longitudinal fibres; ex, excretory system; /, flame cells; g, 

 gut; I.e., lateral nerve cord; m, mouth; m./., muscle fibres; ph., pharynx; t, testis; , 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? 



PIG. 90. Diagram of flame cell, the internal terminus of the excretory tubules, c, cilia lining 

 the tubule;/, special cilia constituting the flame; n, nucleus of flame cell; p, cell processes; v, vacuole 

 or cavity in cell communicating with the capillary tubules (I). 



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



FIG. 91. Diagrammatic sagittal section of Microstomum, showing a chain of four zooids 

 produced by fission, b, brain of the original zooid (the exponents indicating corresponding struc- 

 tures of the more recently formed zo6ids); c, ciliated pit; d, dissepiments indicating different stages 

 in the separation of the zooids; e, eye-spot; wf, 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. 101) ? How 

 unlike? 



