142 PRIMITIVE FORMS OF LIFE 



case of the Annelid Worms, all the phases of this differentiation 

 and of the migration of the nerve cells out of the epidermis, 

 from which they only separate completely by slow degrees, in 

 order to form an independent ring. This differentiation and 

 separation, which early became independent of external 

 excitation, and operative only through heredity, gradually 

 became more and more precocious following the laws of tachy- 

 genesis, as the nervous system assumed greater importance. 

 What fundamentally characterizes the Vertebrates is precisely 

 the large volume of the nervous system in comparison with 

 the rest of the body. The nervous system must accordingly be 

 formed very early, and this is what we find, in fact, in Amphioxus 

 and in the Tunicates which are degenerate forms of the same 

 group. In these animal organisms — and the same is true of 

 all the lower Vertebrates — the nervous system is formed by 

 a modification followed by the invagination of one complete 

 embryonic surface, and these phenomena long precede the 

 formation of the mouth. At the time when the mouth can form, 

 the place it ought to occupy is taken by the already far 

 advanced general outline of the nervous system. But at this 

 particular moment there is formed in Amphioxus, on one side 

 of the body, the first branchial slit establishing communication 

 between the exterior and the cavity of the future digestive 

 tube. The young organism makes use of it as it would make 

 use of the mouth it does not yet possess, but it is obliged for 

 that reason to lie on its side, and, like the soles, to turn this 

 side into a ventral surface ; like them, also, it becomes dis- 

 symmetrical. This dissymmetry is revealed by the encroach- 

 ment by the muscle segments of one side of the body across on 

 to the other, so that each semi-segment of one side is in advance 

 of the other by one half of its length ; by the localization 

 on the side of the body that has become dorsal, of the olfactory 

 pit ; and also by the formation on the same free side of the 

 body, of two series of branchial slits arranged in a curve, and 

 thus betraying the torsion that the body has had to undergo 

 in order to bring over to its free side the branchial slits of the 

 other, which the impurities of the soil would have blocked up. 

 The young organism at this stage swims on its side like the 

 Pleuronectid Fishes. Later it burrows vertically into the sand, 

 and since everything around it is once more symmetrical, it 

 proceeds to repair its dissymmetry. The series of branchial slits, 



