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E. A. ANDREWS. 



areolar tissue and a delicate epidermis underlies the shell and is 

 continuous with the areolar tissue. The internal anatomy and 

 the external modelling is shown by the series of sections, Figs. 

 7, 8, 9, cut along the lines 7, 8, 9 of Fig. 3. The organ is 

 essentially a thick, flat plate with a groove on its posterior face 

 dividing it into a smaller median part that we will call the median 

 mass and a larger external mass, M.M. and E.v.in. in Fig. 7. 



The groove is made much deeper by the fact that a great ridge, 

 R, rises up from the external mass and forms the external 

 boundary of the groove. In the middle of the course of the 

 groove the ridge, Fig. 8, extends toward the middle line of the 



FIG. 8. 



body parallel to the median mass so that the groove is here deep 

 and narrow and opens out more to the median face of the organ. 

 Toward the basal end, Fig. 9, the shallow groove is bounded by 



