Phylum Chordata 585 



units, some paired and some unpaired. In fact there are some forty- 

 named bones that have been identified. Some of these bones have their 

 origin in cartilage and others are laid down as membrane bones, re- 

 sulting from the ossification of connective tissue. Associated with the 

 cranium are the capsules for the protection of the sense organs. The 

 vertebral column is made up of a large number of similar vertebrae. The 

 centra of each vertebra is concave on both the anterior and posterior 

 faces. Between these centra are the remnants of the notochord. Dorsal 

 to the centra is the small neural arch through which the nerve cord 

 passes and an elongated neural spine. Laterally from the centra extend 

 the paired parapophyses. In addition, the vertebrae posterior to the 

 anus have a small hemal arch and spine. The numerous ribs are at- 

 tached to the parapophyses. The visceral skeleton is formed of seven 

 paired arches. The first arch or the mandibular attaches to the skull 

 and becomes the jaw ; the second, the hyoid, is in part a support for the 

 tongue and for the gill covers; the remaining arches support the gills 

 and are the gill arches. Arches three to six have the inner border mar- 

 gined with special spiny structures, the gill rakers, which keep solid 

 materials from entering the gill chamber. The last arch has no gill. 

 The appendicular skeleton consists of the pectoral girdle formed of a 

 number of bones and articulated to the skull ; it is generally considered 

 that there is no true pelvic girdle present. There is a pair of flat bones, 

 however, to which the pelvic fins are attached. 



In addition to these there are small intermuscular bones present 

 between some of the ribs; there are interspinal bones that support the 

 dorsal and anal fin rays. 



The body muscles of the perch are segmentally arranged as 

 myotomes and are in four longitudinal bands. A single heavy band is 

 on either side along the back and there is a thinner band on either side 

 of both the tail and trunk. The myotomes are separated by septa. 



Within the mouth of the yellow perch are many small teeth which 

 function to hold the small aquatic invertebrates which constitute its 

 food. These teeth are strictly prehensile in function. The small tongue 

 at the floor of the mouth functions chiefly as a sensory organ. Posterior 

 to the mouth is the pharynx which commimicates with the gill chamber 

 through four gill slits on either side. The digestive tract continues as 

 the short esophagus which opens into the curved stomach. The stomach 

 passes into the intestine which is somewhat looped and ends at the 

 anus. At the junction of the stomach and intestine are three short 

 pyloric caeca. There is a large liver with a gall bladder and bile duct. 



