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ESSENTIALS OF ZOOLOGY 



at their junction a Cuvierian vein is formed which enters the 

 heart posteriorly. Similar cardinal veins occur on the left 

 side, and they form a left Cuvierian vein to connect them with 

 the heart, but the left posterior cardinal does not reach the 

 caudal vein. The Cuvierian veins, which are transverse veins, 

 are paralleled by a series of transverse vessels posteriorly to 

 them, the anterior of which enter paired vessels which bring 

 the blood from parietal sinuses to the heart. The circulation 

 is simple and primitive, but it is essentially that of the fish 

 and of the embryos and larvae of the higher vertebrates. 



The central nervous system, the dorsal nerve cord, extends 

 from near the anterior end to near the end of the tail, and it is 



enclosed throughout in 

 a neural canal formed 

 of connective tissue. 

 The anterior end is 

 the brain, but it is not 

 distinguished by size 

 from the rest of the 

 cord. Moreover, how 

 much of the cord is 

 brain and how much 

 spinal cord cannot 

 be said, for there 

 is nothing to indicate 



Remnant of 

 neuropore 



Dorsal 

 nerve cells 



Eye spot 



Infundibulum \ Central canal 



Tuberculum 

 postering 



FIG. 66. Amphioxus. Brain vesicle. 

 After v. Kupffer. 



where the one begins and the other ends. The blunt end 

 presents a pigment spot, and after an interval pigment spots 

 occur in the central canal all along the cord ; the posterior 

 end gradually tapers, as does the notochord below it. The 

 front end of the cord is occupied by a cavity formed at the 

 expense of the enclosing nervous wall, and the cavity is con- 

 tinuous with the central canal which runs throughout the rest 

 of the cord. The anterior vesicle represents the forebrain of 

 the Craniates, for it has an infundibular organ on the postero- 

 ventral aspect of its floor. In the young the anterior ventricle 

 of the brain is open to the exterior by the neuropore, but in 

 the adult it is closed. The ectoderm leaves, however, at the 

 spot a blind pit with an opening on the left side, and this pit 

 has been called the olfactory pit. Below this is the pigment spot. 



