DISSECTION OF A BONY FISH. 11 



divided into the operculum proper (composed of several 

 parts) and the branchiostegal membrane, supported by the 

 bony branchiostegal rays, which completes the apparatus 

 below. Connecting the branchiostegal region with the 

 trunk is the narrow isthmus, separating the gill-openings 

 of the two sides. 



Lift the operculum and see the gills. Each is composed 

 of rows of red gill-filaments supported on a branchial arch. 

 Between the successive arches are the gill-clefts. How 

 many are there of these ? Open the mouth and see how 

 the gill-clefts are connected with the posterior part (pharynx) 

 of the cavity. Could you regard them as slits in the wall 

 of a tube ? Notice that each arch contains a solid support. 

 Can you see a red blood-vessel running along each arch ? 



Draw a sketch of the left side of the body, inserting and 

 naming all parts that can be seen from the surface. 



INTERNAL STRUCTURE. 



With scalpel and forceps remove a piece of skin from one 

 side of the fish, exposing the underlying muscles. Notice 

 that these are arranged in chevron-like plates, eacli plate 

 (myotome) extending from back to belly, and being divided 

 into dorsal and ventral portions. Pick among the ventral 

 parts of the muscle-plates. Do you find any ribs ? How 

 are they arranged with regard to the myotomes ? 



Open the fish by cutting with the scissors from just in 

 front of the vent, forward, in the median line, to the 

 pectoral fins, taking care to cut nothing but the body- wall. 

 Make other incisions transverse to the first, so that the body- 

 wall on either side may be turned out like a flap, thus 

 opening up the body-cavity, or coelom, containing the 

 viscera. Without further dissection notice the membrane 

 (peritoneum) lining the cavity. Is it silvery or pigmented ? 



