ARTHROPOD A 269 



Do any nerves arise from the brain? Where distributed? Draw from above. 



Make a diagram of the relation of the digestive tract and nervous system from 

 the side view. 



XI. Excretory Organs. The green glands occur at the base of the head, in front 

 of the mouth. The outlets are at the base of the antennae. 



Make a diagrammatic view of an imaginary cross section of the thorax in the 

 region of the heart, and one of the abdomen, showing the position of the internal 

 organs. Also a diagram of a sagittal section showing relations of all the parts 

 discovered. 



306. Sow-bug (Oniscus, a terrestrial form; or Asellus, a fresh-water Isopod). 

 General Form. Use hand lens and identify: 



Head: size, form, number of segments. 

 Eyes: number and position. 

 Antennules and antennae. 

 Mouth-parts: number and structure. 

 Thorax: number of segments. What variation therein? 

 Abdomen. How many segments ? Proofs? 

 Appendages. 



Remove carefully, mount in water on a slide, and examine with low power, 



a thoracic appendage. Sketch. 

 Examine similarly the other thoracic legs and the mouth parts, and make 



drawings of them arranged in the order of their occurrence. 

 Examine similarly the abdominal appendages. What is their number? 



Sketch? 

 Compare the appendages from the different regions, as to structure, form 



and probable function. Are there any gills? Where situated? 

 What! s the number of segments in the body, if there is a pair of appendages 



to each segment? 



Comparisons. Compare the sow-bug with the crayfish as to the degree of 

 union of head and thorax; the number of segments represented in each of the 

 three regions; the degree of differentiation among the appendages; the mode of 

 respiration; the presence of both exopodite and endopodite; as to food, and habits. 

 Physiology and Ecology. A study and report of the animal's habitat, food 

 habits, methods of motion, sensitiveness to light and to other classes of stimuli, 

 should be made. How does Oniscus behave when touched? Do you find any 

 trace of eggs or young? What facts are to be noted concerning them? 



307. Cyclops. These minute fresh-water Crustacea may be found in almost 

 any pool where aquatic plants are found. They flourish well in aquaria. Select 

 several of the larger specimens with egg masses, one on each side the abdomen. 

 Examine in a watch glass with a little water to which a drop of chloroform has 

 been added. Use low power of microscope. 



General Form. (Study both dorsal and ventral surface.) 

 Cephalo-thorax : 



Anterior portion covered with the carapace. How many segments 

 represented? How can we know that this is not merely the head, or 

 the whole cephalo- thorax? 

 Posterior portion (four free thoracic segments). How is it known that 



these are not abdominal segments? 

 Abdomen: form; number and character of the segments. 



