THE .SMELT 



303 



Fig. 285. - 



■ One of 



(armored fishes) Ganoids — was in former geological times 

 much more abundant than now, l;ut still populates the waters 

 of our Central States. The fifth group is that of the Ijung- 

 fishes (Dipnoans), which includes a few rare fishes that five 

 in the mud. As the l)ony fishes are the best 

 known and most widespread we may take 

 one of them, the l)rown trout (Fig. 284), for 

 an anatomical study. 



The general form of body. — Three regions 

 may be distinguished ; \tz. head, trunk, and 

 tail. The head bears the mouth and many 

 organs of special sense. Its conical form 

 facifitates passage through the water. The 

 head is encased in large, bonj' plates ; the 

 trunk is covered by scales ; and the tail 

 carries a thin membrane. The trunk bears 

 two pairs of appendages and three median 

 appendages in addition to the tail. The 

 plates and scales constitute the external 

 skeleton, which is developed in the skin. 

 The internal skeleton consists of an axial 

 portion replacing the chorda. This is made 

 up of dice-box (or hour-glass) shaped bodies, 

 each of which bears above a Ijony arch, which protects the 

 main or dorsal nerve and supports the median fin (Fig. 285). 

 Below, each bears a pair of slender bones, the ribs in front of the 

 anus and the ventral arches loehind. The brain-case consists of 

 numerous bones, some of which arise in the skin and others be- 

 long to the axial skeleton (Fig. 2S6). Here on the head skeletal 

 parts of widely different origin come together into one cranium, 

 and even form parts of one and the same bone. In our own skull 



the anterior verte- 

 bra; of the tail of 

 the trout. C'.V, the 

 centrum; N.A, 

 neural arch ; jV. 

 SP, neural spine ; 

 H.A, h^mal arch 

 {tjeeause toward 

 heart) ; R, ribs. 

 From Parker and 

 Haswell. 



