ARTHROPODA. 265 



Make a sketch of the entire tract from a side view, showing in what 

 part of the carapace each portion is. 



IX. Muscular System. How is the abdomen flexed and how extended? 

 How do the muscle fibres run? To what attached? Are they plain or 

 striate? How are the appendages worked? Split open the segments of the 

 chela. 



X. Nervous System. (If the time is short a demonstration may be 

 made by the teacher, preferably with a lobster.) 



Remove the intestine, and cut carefully through the muscles in the 

 median line until the white ventral nerve-chain is uncovered. Follow it 

 forward to the head, cutting away the covering plates in the thorax. 



How many swellings (ganglia) in the abdominal region? Relation to 

 the segments ? Where do nerves arise ? 



Thoracic ganglia : number and relation to appendages ? 



Subcesophageal ganglion; circumcesophageal connective. 



Supraoesophageal ganglion (brain). 



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

 above. 



Make a diagram of the relation of the digestive tract and nervous sys- 

 tem 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 rela- 

 tions of all the parts discovered. 



. 299. Sowbug (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 is the number of segments in the body, if there is a pair of 



appendages to each segment? 



