INVERTEBRATE ZOOLOGY. 113 



The sexes may be distinguished as follows : Male, smaller than female, with 

 darker colored body, terminating posteriorly in a large romided knob. Female, 

 often twice the size of the male, with large heavy body terminating in four 

 spines, which may approximate to form a single sharp spine for boring holes 

 in the ground to deposit the eggs. The sexes may be indifferently used as 

 specimens, except in the sections on the reproductive system. For immediate 

 use, or for the study of external parts, specimens may be dropped alive into 

 collecting bottles containing 70-80 ^ alcohol. This will not penetrate the inte- 

 rior, however, suflEiciently to preserve the inner organs. Benzine is the quick- 

 est and simplest killing agent and may be used whenever a specimen is to be 

 dissected at once. Dried specimens are often useful in the study of external 

 parts. 



88. Symmetry. Form, Orientation: The body is 6(7«fe/'«/, i. e. it maj'be 

 divided by one and only one plane, into two sjTiimetrical halves. Locate this 

 plane. The halves are termed rigitt and left. Imagine the animal resting on 

 a level surface in the natural position, and pass a plane through it, parallel to 

 the level surface and perpendicular to the first plane. Are these halves equal 

 and symmetrical 'i What are the characteristics of each, i. e. color, general 

 shape, thickness of shell, etc. V What external environments may have pro- 

 duced the difference ? 



The upper half is termed dorsal, the lower, ventral. Pass a })lane through 

 the middle of the body, perpendicular to the two preceding. This divides the 

 bod}' into two halves, a» fervor ■Axid posterior. The above terms are also ap- 

 plied relatively, thus the inner pair of wings are ventral to the oiiter, although 

 both lie in the dorsal half. The exterior is hard, formed of plates which over- 

 lap each other, allowing the necessary motion, as in a suit of mediaeval 

 armor. This hard exterior is really the skeleton, and is termed exo-skeleton 

 in distinction from an internal, or endo-skeleton. The joints overlap each 

 other in the direction from which injuries are liable to come, or in the direc- 

 tion of the most usual motion, cf. the joints of the posterior half of the body. 



89. Metamerism. Body regio)is. The body is divided transversely into 

 more or less separate joints or segments. The segments are also termed meta- 

 meres or somites, and an animal possessing thein is said to be metameric or 

 segmented, cf. oj'^ster and earth-worm. In a simple metameric animal, the 

 segments are all alike and distinct from each other, while in higher forms 

 segments may be suppressed, over developed, united together and modified in 

 many ways. This condition is considered higher because it has developed 

 from the first, and is ft)r the purpose of doing certain things better : thus the 



