THE PHARYNGOBRANCHII. 



119 



in the earliest embryonic stage only, in other Vertebrata. 

 The blood brought back from the body and from the ali- 

 mentary canal enters a pulsatile cardiac trunk, which runs 

 along the middle of the base of the pharynx, and sends 

 branches up on each side. The two most anterior of these 

 pass directly to the dorsal aorta ; the others enter into the 

 ciliated bars which separate the branchial slits, and, there- 

 fore, are so many branchial arteries. Contractile dilata- 

 tions are placed at the bases of these branchial arteries. 

 On the dorsal side of the pharynx the blood is poured, by 

 the two anterior trunks, and by the branchial veins which 

 carry away the aerated blood from the branchial bars, into 

 a great longitudinal trunk, or dorsal aorta, by which it is 

 distributed throughout the body. ^ 



Notwithstanding the extremely rudimentary condition of 

 the liver, it is interesting to observe that a contractile trunk, 

 which brings back the blood of the intestine, is distributed 

 on the hepatic sac after the manner of a portal vein. The 

 blood is collected again into another contractile trunk, which 

 represents the hepatic vein, and is continued into the cardiac 

 trunk at the base of the branchial sac. The corpuscles of 

 the blood are all colourless and nucleated. 



The skeleton is in an extremely rudimentary condition, 

 the spinal column being represented by a notochord, which 

 extends throughout the whole length of the body, and ter- 

 minates, at each extremity, in a point (Fig. 28). The in- 

 vestment of the notochord is wholly membranous, as are 

 the boundary-walls of the neural and visceral chambers, 

 so that there is no appearance of vertebral centra, arches, 

 or ribs. A longitudinal series of small semi- cartilaginous 

 rodlike bodies, which lie above the neural canal, represent 

 either neural spines or fin-rays (Fig. 28, B, b). Neither is 

 there a trace of any distinct skuU, jaws, or hyoidean appa- 

 ratus ; and, indeed, the neural chamber which occupies the 

 place of the skuU, has a somewhat smaller capacity than a 

 segment of the spinal canal of equal length. 



There are no auditory organs, and it is doubtful if a 

 ciliated sac, which exists in the middle line, at the front 



