42 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



since those that follow and which make up the greater part of the spinal cord were 

 acquired after the evolution of vertebrates from arachnids had taken place. 



In other words, the ancestors of vertebrates were animals provided with a 

 comparatively small number of neuromeres, 21 ±, most of which had already 

 been consolidated into a complex brain of the vertebrate type. One of the im- 

 portant events in the early evolution of the new or vertebrate type was the rapid 

 increase in the number of metameres by the regular process of apical growth. 

 The new metameres formed a new trunk or body, while nearly the whole of the 

 old arachnid trunk (head, thorax, and abdomen, 14-16 metameres) was still 

 further consolidated to form the head of the new type. The whole process thus 



FxG. 35. — Diagrams to explain the probable relations between the structure of a trochosphere and the early 

 embryonic stages of a primitive arthropod; A, Trochosphere in mercator projection, seen from the neural, or sub- 

 umbrella surface; C, same from the side seen as a solid object; B, early stage of an arthropod embryo, seen in 

 mercator projection; D, same seen as a solid object, from the side. In A and C, the circumoral area, with its 

 system of radial and circular nerves, forms a part of the sub-umbrella of the trochosphere. In B and D this area 

 is supposed to be infolded, giving rise to the proximal portion of the stomodffium, from which the system of stomo- 

 dseal nerves and ganglia arise. The ancestral coelenterate body, according to this interpretation, is represented in 

 the arthropod embryo by the procephalic lobes and stomodasum; the arthropod trunk, with its lateral and 

 median nerve cords, is a new formation, arising as a local outgrowth from the ancestral coelenterate body, or from 

 the procephalic lobes of the arthropod embryo. On the aboral surface of the trochosphere is the area of yolk de- 

 posit and the "closing in" point, a pauperitic, degenerative region that is called the cephaUc navel. 



presents a striking analogy to the way in which the primitive body of segmented 

 animals was formed as a new outgrowth from the body of its ccelenterate ancestor, 

 which then became the head of its descendant. (Fig. 35.) 



********* 



II. The Stomod^al Nerves. 



We recognize two distinct systems of nerves in segmented animals. One 

 belongs to the stomodseum, and probably represents the remnants of the cir- 

 cular and radial sub-umbrellar nerves of a coelenterate-Hke ancestor; the other 

 consists of longitudinal and transverse nerves that developed in the tentacle-like 

 out-growth that gave rise to the body of the new animal. (Fig. 35.) 



The stomodseum is looked upon as representing, in part, the infolded sub- 

 umbrella. When invaginated, it carried with it the primitive system of circum- 

 oral nerves, which then arise as circular and longitudinal nerves from the walls 

 of the stomodaeum. The outermost circular nerve (prototroch nerve( ?)), is repre- 



