106 THE ANATOMY OF VEETEBRATED ANIMALS. 



raentary canal eaters a pulsatile cardiac trunk, which rung 

 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, therefore, 

 are so many branchial arteries. Contractile dilatations 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 longi- 

 tudinal 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 repre- 

 sents the hepatic vein, and is continued into the cardiac trunk 

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

 are all colorless 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 terminates, at 

 each extremity, in a point (Fig. 28). The investment 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 rod-like bodies, which lie above the 

 neural canal, represent either neural spines or fin-rays (Fig. 

 38, B, b). Neither is there a trace of any distinct skull, jaws, 

 or hyoidean apparatus ; and, indeed, the neural chamber, which 

 occupies the place of the skull, 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 part of the 

 cephalic region (Fig. 39, a), ought to be considered as an olfac- 

 tory org'an. 



The myelon traverses the whole length of the spinal canal, 

 and ends anteriorly without enlarging into a brain. From 

 its rounded termination nerves are given off to the oral region, 

 and to the rudimentary eye or eyes (Fig. 26, b, c). 



According to M. Kovvalewsky,* who has recently studied 



* "Memoirca de rAcailemie Imperiale dea Sciences de St. Petersburg," 

 1867 



