OF FISHES. 229 



results from what is given by Owen, inasmuch as the former 

 represents the currents as passing from The heart (//) partly for- 

 ward into the head, and in part through the branchial vessels (6, 

 b\ br) upward to join the dorsal vessel (7i 4 , h?) bearing the cur- 

 rent from the head toward the tail, and from the latter re- 

 turning to the heart in the lower median vessel, (A 5 ,) which 

 passes along the inferior side of the intestine. 



The vertebral column or spine is represented by a mere cord 

 (v, v l ) of jelly-like matter, which extends along the middle of 

 the back, from the front (/) to the posterior end of the tail. It 

 is enclosed in a membranous sheath, (vs,) within which it lies so 

 loosely that it can be taken out entire by simply opening a gash 

 along the back. It usually goes by the name of the notochord, 

 and corresponds with the first rudiment of the spinal column of 

 the embryo of higher animals.* 



The nervous system consists of a main cord (nr, nr l ) and 

 numerous branching prolongations which project right and left 

 from its sides. At its anterior end (nr l ) there is no sensible 

 dilatation which corresponds to the brain. The nerves of the 

 eye (o) and nose (n) arise from near its rounded end in the same 

 simple way as the other less specialized nerves (nv) which spring 

 up near them and branch in the head. 



The reproductive system is a mere elongated oval mass (of) 

 attached to the upper median line of the general cavity, just 

 behind the branchial chamber. The eggs reach the outer world 

 by dropping from the ovary into the visceral cavity and thence 

 passing out through the abdominal pore, (ap,) an aperture which 

 lies in the lower side just behind the heart. 



If now I have given you a clear understanding of the relation 

 and nature of the organs of the Lancelet, there will be no dif- 

 ficulty in comprehending, by the help of a few words of explana- 

 tion, the organization of one of the highest of the Vertebrates. 

 With the preliminary knowledge of the simpler structure of the 

 one, the apparently puzzling complication of the other may be 



* See the embryology of the Turtle, in chap, xvn., where this body is called 

 the chorda dorsulls. 



