PLATE 4 



EXPLANATION OF FIGURES 



9 A median section of a test vesicle showing its structure and its position 

 in the substance of the tunic. Test cells are shown in various positions, some 

 located near the surface of the mantle, some above and to the right of the test 

 vesicle, and others in the tunic epithelium. Some contain pigment granules, 

 some are without granules. 



10 A portion of a transverse section through the posterior part of the eye. 

 It is reproduced to show especially the four visual rods which are cut trans- 

 versely in the tangential section of the pigment cup and the extent and struc- 

 tures of the visceral ganglion. 



11 A portion of a transverse section in which a visual rod of one of the retina 

 cells is cut lengthwise. Two only of the series of lenses are in the plane of the 

 section. 



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