112 LAliORATOIiV ST ir DIES IN ZOOLOGY 



External Features. — Observe carefully the form of the body 

 with a view to expressing verbally not only its nature but also its 

 adaptation to a darting mode of locomotion through a liquid 

 medium. Hold the fish on a level with the eyes, the tail pointing 

 straight away from the observer's face. What is the nature of the 

 visible surface? The body may be recognized as possessing 

 head, trunk, and tail regions. The scales are flat and imbricated 

 and are arranged in diagonal rows. 



The mouth is terminal in location. On the dorsal surface of the 

 head immediately in front of the eyes is a pair of small openings 

 on each side. These are the nostrils. The two openings of the 

 same side lead into a common nasal cavity. Water enters by one 

 aperture and leaves by the other. Look for the eyelids. Only 

 that part of the ear mechanism corresponding to the frog's inner 

 ear is present in fishes. Accordingly, there is no external indica- 

 tion of the presence of this organ, which is located just caudad of 

 the dorsal portion of the eyes. 



On the sides of the head note the gill cover or operculum com- 

 posed of fiat bones overlaid by a thin layer of soft tissues. 

 By elevating the operculum the gills and gill slits may be seen. 

 Attached to the ventral margin of each operculum is a membrane, 

 the hranchiostegal membrane, supported by skeletal elements 

 called hranchiostegal rays. To see these the membrane must be 

 spread. 



In order to study the respiratory structures to a better advan- 

 tage remove the operculum of the left side. Expose the mouth 

 cavity and pharynx by cutting through the angle of the jaws and 

 extending the cut across each of the supporting arches of the 

 gills. Divaricate the parts thus severed. 



The supports of the gills are known as gill arches. They, 

 together with the jaws and the tongue support (hyoid arch), 

 constitute the visceral skeleton. Between the gill arches are the 

 gill slits. The pharynx or posterior portion of the exposed cavity 

 can be distinguished from the oral or mouth cavity by the 

 presence of the gill arches and gill slits in its walls. 



Of the five gill arches only the first four actually bear gills. 

 The dorsal ends of the arches of each side are attached to the 

 undersurface of the brain case, while the ventral ends articulate 

 with a series of bones in the midventral line. This connecting 

 series of bones constitutes what are known as the hasibranchials. 

 The inner surface of each of the first four gill arches bears spine- 



