24 LABORATORY WORK. 



What is the position of the mouth? See that it has 

 a bony framework, the upper jaw being composed of a 

 premaxillarij in front, and behind this a maxillary which 

 when the mouth is open slides over the dentary or lower 

 jaw. Do any of these bones bear teeth? Open the mouth 

 and examine the tongue. How much can it move? Can 

 you find teeth anywhere inside of the mouth? Feel with 

 a pin. 



How many nostrils, and where situated? Probe with a 

 bristle. Do they communicate with the mouth? Can you 

 find any ears? 



THE BRANCHIAL APPARATUS. Find the gill-opening, a 

 crescentic slit on the side bounding the head behind. In 

 front of it is the gill-cover or operculum, which may be 

 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. 



