154 



AN INTRODUCTION TO ZOOLOGY 



structures difficult to determine in a living animal. In order to 

 study Planaria successfully in the laboratory, the soft, contractile 



body is usually placed on a 

 slide, and then pressed out 

 slightly with a cover glass. 



A pair of eye spots (Fig. 76, /) 

 are present on the dorsal sur- 

 face near the anterior end. 

 The mouth is in a peculiar posi- 

 tion near the middle of the 

 ventral surface. From it the 

 muscular proboscis (j) may 

 extend. Posterior to the 

 mouth is a smaller opening, 

 the genital pore (5). The sur- 

 face of the body is covered with 

 cilia, which propel the animal 

 through the water. This is 

 not the only method of loco- 

 motion, since muscular con- 

 traction is also effective. 



Structure. A study of the 

 structure of the adult and of 

 the early embryonic stages 

 shows Planaria to be a triplo- 

 blastic animal possessing the 

 three germ layers, ectoderm, 

 mesoderm, and entoderm, from 

 ovary; i\ 22, wn ^ cn several systems of or- 



i s , branches of intestine ; In, lateral gans have been derived. 



nerve ; m, mouth ; ph, pharynx ; There are well-developed mus- 



nervouSj digestive, ex- 

 , and "productive 



mon genital pore. (From Lan- systems; these are constructed 



kester's Treatise after V. Graff.) in such a way as to function 



FIG. 77. Anatomy of Planaria. 

 en, brain ; e, eye ; 



od, oviduct ; t, testis ; w, uterus ; 

 v, yolk glands ; vd, vas deferens ; 

 , penis ; , vagina ; , <j> , com- 



