viding the body into approximately equal right and left 

 halves. This middle plane is called the meson (/xeo-o^, mid- 

 dle). From meson are derived the adjective mesal and the 

 adverb mesad. 



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 (CKTOS, 

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

 forms of these terms are ectad and entad. 



EXAMPLE. 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. 

 The hairs or spines on the body-wall project ectad. 



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 dorsal, 

 ventral, cephalic, caudal and the two lateral. 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 describ- 

 ing appendages of the body (legs, wings, etc.) the position of 

 parts may be referred to the two ends of the appendage by 

 use of the terms proximal and distal. Proximal indicates 

 nearness to the end of the appendage which is attached to 

 the body ; distal, to the end which is free. From these ad- 



* Cases occur where it is desirable to speak of an aspect which looks in a 

 direction between two of the cardinal directions. Thus we speak ot the lines or 

 spots on the latero-dorsal aspect of a larva. 



