x PRACTICAL DIRECTIONS 481 



H. Side Dissection. It is very instructive to supplement 

 and recapitulate your work on the anatomy of the dogfish 

 by dissecting another specimen from the side (compare 

 Fig. 117), as in the case of the crayfish. Dissect off the 

 skin from the left side for an inch or so and observe the 

 myomeres and myocommas. Cut open the abdominal cavity 

 as before, very slightly to the left side of the middle line. 

 Continue to cut forwards through the pectoral arch and 

 backwards through the pelvic arch ; the arteries may be 

 injected at this stage. Then dissect off the left half of each 

 arch with the corresponding fin, and cut away the left body- 

 wall. Cut through the skin in the mid -dorsal line : and 

 then, working from the left side, remove the left half of the 

 skull and visceral arches, as well as the left side of the 

 vertebral column, so as to expose the brain and spinal cord. 

 Remove the left kidney (and left spermary if your specimen 

 is a male ; left oviduct if it is a female). Pin down under 

 water, and tidy your dissection so as to reduce it to a neat 

 longitudinal section, in which all the unpaired soft organs 

 are left intact. Without tearing the mesentery, pin out 

 the liver, stomach, and intestine beyond the ventral limits 

 of the body-wall, so that the other abdominal organs are 

 not hidden. Follow out the relations of the various organs 

 as before, and sketch your dissection. 



It is important at this stage to refresh your memory of 

 the anatomy of the frog, and to compare it with the dogfish, 

 by making a similar dissection of it from the side (compare 

 Fig. 7). Sketch. 



