118 



Comparative Morphology of Chordates 



VENOSUS 



COMMON 

 CARDINAL- 

 VEIN 



Fig. 323. A diagram of the primitive (fish) heart, as seen in a median longi- 

 tudinal section. Anterior is to the right. The course of blood in the heart (indicated 

 by arrows) takes the form of a letter S. (After Keith. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



which lie just under the notoehord. The arches may be cartilaginous 

 or bony. They are, however, much more strongly developed than the 

 neural arches of cyclostomes. 



The maximum number of pharyngeal clefts occurs in a shark, 

 Heptanchus (Fig. 329 A), having a pair of spiracles and seven pairs of 

 gill-clefts. Most adult fishes have no spiracles and only five pairs of 

 gill-clefts. In some cases there are fewer than five. The highly charac- 

 teristic visceral skeleton, cartilaginous or bony, is developed around 

 the pharynx, consisting of arches alternating in position with the 

 pharyngeal clefts (Fig. 120). 



Most fishes have a breathing valve which prevents exit of water 

 from the mouth when the pharyngeal wall contracts to force water out 

 via the gill-chambers. Just inside the oral aperture are a dorsal and a 

 ventral inwardly projecting fold of the oral lining (Fig. 322). Pressure 

 of water against these flaps from behind forces them forward so that 

 they meet and occlude the oral passage. 



In most fishes the heart consists of two chambers, a receiving 

 chamber, the auricle, and a pumping chamber, the ventricle (Fig. 323). 

 In lungfishes there are two auricles, the right one receiving blood from 

 the general circulation and the left receiving blood from the lungs. 

 The two auricles open into a common single ventricle. 



The nervous organs of fishes are arranged on the same plan as in 

 all other vertebrates. Of all organ-systems, none is more conservative 



