378 THE CRAYFISH chap. 



sternal and ventral abdnminal arteries (p. 375) will be seen, the sternal 

 artery passing between the connectives joining the fifth and sixth 

 postoral ganglia. 



2. The brain gives off nerves to the eyes and the two pairs of 

 feelers ; the suboosophageal ganglion supplies the mandibles and 

 four following pairs of appendages with their segments. Each of 

 the other ganglia supplies one segment (with its appendages) only, 

 except in the ease of the last or 6th abdominal ganglion, which 

 gives off nerves to the telson. (A small anterior visceral nerve, 

 arising frqm the brain and oesophageal connectives and supplying the 

 fore-gut, will have been removed, and a small posterior visceral 

 nerve, supplying the hind-gut, arises from the last abdominal 

 ganglion. ) Sketch. 



\ . The antennary or green-glands are situated just behind the bases 

 of the large feelers. Blow through the renal aperture of one side (p. 373) 

 and note the duct and urinary bladder on the dorsal side of the gland. 

 Then cut through the duct,' remove the whole gland, and examine under 

 water. Sketch. 



D. The Appendages. — Remove the appendages of one side, 

 beginning with the last, one by one, cutting through the articular 

 membrane with a scalpel, and then taking hold of the basal joint with 

 the forceps and pulling- the appendage away. Work through the de- 

 scription on pp. 350-353 and sketch typical appendages from each region. 



Note the delicate paragimtha behind the mouth and the lahriim 

 in front of it (p. 356). 



The arrangement of the joints and muscles of the limbs can be well 

 seen by examining the large first leg or cheliped. Note that each joint 

 works in a different plane, and then cut away the exoskeleton 

 from one side of the two or three distal podomeres, so as to 

 expose the muscles (compare Fig. 85). Then remove these, and note 

 their chitinous tendons. Observe that the flexor muscle and its tendon, 

 which moves the pincers, is much larger than the extensor muscle. 



E. Sensory Organs. 



1 . Tactile organs. Snip off some setse from the body or appendages. 

 Examine under the microscope. Sketch. 



2. " Olfactory'" organs. Examine the outer flagellnm of the 

 antennule under the low power, and note the tufts of spalula-like 

 olfactory setie on the ventral surface. Sketch. 



3. Auditory organ. Carefully cut away the convex ventral side o( 



