ARTHROPODA. 271 



Compare the nervous system of the grasshopper part by part with 



that of the cray-fish. 



Make diagrammatic representations of imaginary cross-sections 

 through thorax and abdomen showing the relation of the different 

 structures : likewise of a sagittal section. 



The cricket or cockroach may be substituted for or compared with 

 the grasshopper. 



304. Supplementary Laboratory and Field Work. It 



is perhaps inexpedient for students in an elementary course 

 to make dissections of other representatives of the Arthropoda, 

 but the common air-breathing forms are so numerous, so 

 varied, and have such interesting habits and histories, that 

 they may profitably be used as a basis -for individual field and 

 laboratory work and to serve in the comparison of homologous 

 organs in related groups. The following outlines are sug- 

 gestive rather than exhaustive. 



I. Make a table in which can be displayed the points of con- 

 trast between the cray-fish, the grasshopper, the " June bug " 

 (or other beetle), the squash-bug or the cicada ("locust "), 

 the butterfly, the wasp, the fly, the spider, and the centipede, 

 in the following particulars : 



1. The regions of the body; head, thorax, and abdomen; 

 their degree of development and separateness. 



2. The number of segments in the body, and the clearness 

 with which they are manifest. 



3. The degree of diversity shown by the segments in the 

 various parts of the body. 



4. The points of structure which the various segments 

 possess in common. 



II. Make a similar table, for the same animals, of the 

 appendages. 



1. Head appendages: antennae; mouth parts, number and 

 kinds. 



2. Thoracic appendages: 



Legs : number, position, kinds, joints, special adaptations 

 to special work. 



