PLATE 1 



EXPLANATION OF FIGURES 



1 Transverse section involving the fundus of ciliated pit. GN , nucleus of 

 gland cell; GC, cytoplasm of gland cell; PW , syncytial wall of pit; M, muscles 

 of pit. In the reconstructions (figs. 6 and 7) this section is shown at A, A' . 

 X 1500. 



2 Section at B and B' in reconstructed figures 6 and 7. GC, cytoplasm of 

 gland cell ; GP, pores of gland cell ; PW, syncytial wall of pit, with two nuclei shown 

 in section; A'^, nerve of pit; M, muscles of pit. X 1500. 



3 Section at C and C in reconstructed figures 6 and 7. At this level the sen- 

 sory cytoplasmic ridge of wall of the pit is structurally different from the cyto- 

 l)lasm of the general wall of the pit. SR, sensory ridge. N, nerve of pit. 

 X 1500. 



4 Section at D and D' in reconstructed figures 6 and 7. The lumen at this 

 level is greatly narrowed posterior to the sensory ridges, SR., and remains wide 

 and conspicuously ciliated anterior to these ridges. X 1500. 



5 Frontal section of a pit. Taken from about the level, G, in reconstructed 

 figure 7. GP, pores of gland cell opening into lumen of pit. X 1500. 



6 Reconstruction, based upon sections figured at 1, 2, 3, and 4. Anterior 

 aspect of pit of right side. A, B, C, D, E, F represent levels of sections; M, mus- 

 cles of pit; GC, gland cells; GP, pores of gland cell, which lead into lumen of pit; 

 X 2000. 



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