Construction of the terms used. — Excepting the nouns 

 meson, dorsimeson, and ve?itrimeso?i , 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, and dextro-sinistral. 



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 directio?i or headward ; this is the direction 

 indicated 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. 



The dextral direction or towards the right. 



The sinistral direction or towards the left. 



The ventral direction or bellyward ; 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, 

 dextral, sinistral, ventral, and dorsal are cephalad, caudad, 



