Meson, mesal, and mesad — Frequently the position or 

 direction of parts is referred to an imaginary plane dividing 

 the body into approximately equal right and left halves. 

 This middle plane is called the meson (fxearov, middle). 

 From meson are derived the adjective mesal and the adverb 

 mesad. 



Dorsimeson and ventrimeson. — If it is necessary to 

 refer to the lines constituting the dorsal and ventral borders 

 of the meson, these are designated as the dorsimeson and 

 ventrimeson respectively. 



Example. — The wing-covers of a beetle meet without overlapping 

 on the dorsimeson. 



Ectal, ectad, ental, and entad. — It is often necessary, 

 especially in the study of internal anatomy, to compare parts 

 with relation to their nearness to or remoteness -from the 

 surface of the body. For this purpose the terms ectal (ckto's, 

 without) and ental (ivros, within) are used. The adverbial 

 forms of these terms are ectad and entad. 



Examples. — The principal muscles of an insect are attached to 

 the ental surface of the body-wall, or to parts of the body- wall which 

 project entad. 



Aspects of the body. — In describing animals it is often 

 desirable to specify that part of the body which looks in a 

 certain direction. For this purpose the term aspect is used 

 combined with an adjective indicating the direction in which 

 the surface in question looks. 



Examples. — Dorsal aspect, ventral aspect. 



Six aspects of the body are recognized ; these are the two 

 lateral (dextral and sinistral), dorsal, ventral, cephalic, and 

 caudal.* The fact that the outlines of the body of an animal 

 are more or less curved does not interfere with the practical 

 application of the above terms. 



Proximal, distal, proximad, and distad. — In describing 



* Cases occur where it is desirable to npeak of an aspect which looks in a direc- 

 tion between two of the cardinal directions. Thus we speak of the lines or spots 

 on the latero-dorsal aspect of a larva. 



