ARTHROPODA 275 



Ventral nerve cord. Is it single or double? 

 Ganglia; number, and relation to the segments. 

 Nerves; origin and distribution. 

 Trace forward into the thorax and head. 

 Ganglia; number and position. How connected? Is there any 



portion dorsal to the digestive tract (brain)? 

 Nerves. 

 Compare the nervous system of the grasshopper part by part with that 



of the crayfish. 

 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 grass- 

 hopper. 



311. 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 

 and library work and to serve in the comparison of homologous 

 organs in related groups. The following outlines are suggestive 

 rather than exhaustive. 



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

 trast between the crayfish, 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 : 



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

 kinds. 



