76 Journal of Entomology and Zoology 



The distortion may be eliminated in either of two ways. The 

 first and best is to set the orientation guide at an agle of about 45 

 degrees from the axis of the object and cut the sections at right 

 angles to the orientation guide and hence obliquely to the specimen. 

 The reconstruction lines are drawn in the same way except that the 

 projection of the orientation line is allowed to fall in the same place 

 each time, thus eliminating the displacement and consequent distor- 

 tion. It is hard, however, to get a clear idea of the relations of 

 parts from oblique sections. The reconstruction does not show 

 any more than one that follows the first method, and each series is 

 good for reconstructing only one aspect. 



The other way of getting rid of the distortion is to insert at the 

 place of proper magnification in the cone of light rays from the 

 projector a lense of sufficient curvature to refract the rays into a 

 parallel bundle. By tilting the drawing board at a proper angle to 

 this bundle the field will be caused to fall in such an ellipse as to 

 eliminate any distortion. This angle is one whose tangent equals 

 the displacement divided by the product of the thickness of the sec- 

 tion times the magnification in diameters. 



Occasionally wrinkling of the section in cutting or in mounting 

 occurs and renders it necessary to disregard the orientation guide. 

 It is easy, however to put the section in approximately its right 

 place and to check it up by the next section. In reconstructing sym- 

 metrical specimens where there is a clearly marked axial line it is 

 often possible to dispense with the orientation guide and to place 

 the successive sections from landmarks which they themselves bear. 



ANATOMY 



The brain (Pig. 1, 2, 4) is a very complex structure. Topo- 

 graphically it is divided into two parts, the dorsal and ventral by 

 the central mass of muscle and blood vessels (Figs. 5, 6, 7; q), 

 which tissue, going forward from between the central part of the 

 brain and the visceral ganglion, pinches out into a muscular sheet 

 at either side and separates the six dorsal lobes from the eight to 

 ten ventral lobes or branches of the fore part of the brain. The 

 brain is symmetrical and is divided into lateral halves by a septum 

 which continues as a canal through the main brain (Fig. 6; w). 



