Construction of the terms used.— Excepting the noun 

 meson, the terms used in this work for denoting the position 

 and direction of parts are either adjectives or adverbs. 



The adjectives end in al^ as dorsal, ventral, and mesal ; 

 the adjectives cephalic and intermediate are exceptions to 

 this rule. 



The adverbs are formed by substituting for the adjective 

 ending the ending ad, the Latin equivalent of the English 

 suffix ward. Thus from the adjectives dorsal, ventral, and 

 mesal, are formed the adverbs dorsad, ventrad, and mesad. 



In forming compound words indicating position or direc- 

 tion, the vowel o is substituted for the termination of the first 

 member of the compound, as dorso-ventral, caudo-cephalic. 



The six cardinal directions.— There are six principal 

 directions to which the position and direction of the parts of 

 a bilaterally symmetrical animal, like an insect, are com- 

 monly referred ; these are as follows : 



The cephalic direction or head ward ; this is the direction in- 

 dicated by a line drawn from the center of the animal to the 

 head. 



The caudal direction or tailward ; this is the opposite of the 

 cephalic direction. 



Two lateral directions, or towards the right and towards the 

 left. 



The ventral direction or belly ward ; this is the direction 

 indicated by a line drawn from the center of the body to the 

 ventral surface and forming a right angle with each of the 

 preceding directions. 



The dorsal direction or backward ; this is the opposite of 

 the ventral direction. 



The adverbial forms of the adjectives cephalic, caudal, 

 lateral, ventral and dorsal are cephalad, caudad, laterad, ventrad, 

 and dorsad. Thus a part which extends in a cephalic direc- 

 tion may be said to extend cephalad. 



