Phylum Chordata 583 



Subclass II. Choanichthyes. With internal nostrils and fleshy lobed fins. 



Order 1. Crossopterygii. With typical lobed fins; forms which are pos- 

 sibly ancestral to land vertebrates. Example : Latimeria. 



Order 2. Dipnoi. Fish having air bladder modified for respiration. The 

 lungfish : Lepidosiren of South America ; Protopterus of central Africa. 



The Anatomy of the Perch, Perca flavescens. — While there is 

 great variation in both the external and internal anatomy of fish, the 

 yellow perch, Perca flavescens, may be studied as an example. This 

 fresh-water species is common throughout the northeastern United 

 States and attains about a foot in length when adult. It is an important 

 food fish. 



Externally three distinct regions can be recognized : the head, 

 trunk, and tail. In the head region, the large terminal mouth with its 

 many fine teeth is the most conspicuous feature. Above the mouth are 

 the openings of the paired double olfactory sacs. Slightly posterior to 

 the nares are the two large laterally placed eyes. At the posterior por- 

 tion of the head are the large opercula which cover the gills. The skin 

 surface is covered with a large number of ctenoid scales; these dermal 

 scales overlap like shingles and form the exoskeleton of the animal. 

 There are several fins present : the paired pelvic and pectoral fins, 

 two unpaired dorsal fins, the unpaired anal fin, and the single large 

 caudal fin or tail. The fins are supported by the fn rays or spines. The 

 anterior dorsal fin and the anterior portion of the anal and pelvic fins 

 are supported by spines. These are unbranched and unjointed. The 

 other fins or portions of fins are supported by fin rays which are jointed, 

 branched, and soft. Forward movement of the perch is due chiefly 

 to lateral lashings of the tail fin and portions of the trunk. During very 

 slow swimming, the paired lateral fins move back and forth in short 

 strokes. With the caudal fin, these latter are important in steering. 

 The paired lateral fins also aid in up and down movements. The anal 

 and dorsal fins are important for maintaining equilibrium. The verte- 

 brae extend only slightly into the dorsal lobe of the tail fin : this type 

 is known as the homocercal tail (Fig. 197), with the dorsal and ventral 

 lobes being equal in size. Along the sides of the body the lateral line is 

 evident. This is made up of a row of small sensory pits which respond 

 to changes in direction and flow of water as well as to water pressure. 

 The internal bony skeleton of the perch is complex and consists 

 of many individual bones. The axial skeleton is made up of the cranium, 

 vertebrae, and ribs. The cranium is composed of a number of bony 



