Descriptive Biology 107 



Its primitive character is evidenced by the superficial net 

 work of nerve fibres extending all over the body, and resem- 

 bling in these respects the nervous system of a sea anemone. 

 Along the mid-dorsal and mid-ventral lines this network is 

 thickened to form definite nerve cords, thus relating the 

 animal to the invertebrates with their ventral nerve cord on 

 the one hand and on the other to the vertebrates, whose 

 central nervous system is dorsal. This latter resemblance is 

 still further enhanced by the hollow character of the dorsal 

 cord in the collar region (at least in young animals) and its 

 separation from the surface and deeper situation in the body 

 in this region. The dorsal and ventral cords are joined by 

 a ring surrounding the pharynx at the base of the collar 

 similar to that of an annelid. 



There are then certain points of fundamental importance 

 which are common both to the tunicate and Balanoglossus, 

 and to the vertebrates. These are the notochord, the gill slits 

 and the dorsal nerve cord. The opposite position of the 

 mouth in the two former (dorsal in the tunicate and ventral 

 in Balanoglossus) introduces an element of uncertainty, and 

 indeed the origin of the vertebrate mouth is a question of 

 great difficulty. It is improbable that vertebrates can claim 

 either the tunicate or Balanoglossus as a direct ancestor. In 

 fact "direct ancestors" in the animal world are at a premium. 

 In the very nature of the case, if living things are labile and 

 not stabile, this must be so. Otherwise all organisms would 

 become "stand patters" and evolutionary progress cease. 

 But, regardless of direct ancestry, both of the organisms dis- 

 cussed show distinct vertebrate affinities and indicate the way 

 which the latter have gone in their advance. 



Further on the path of vertebrate development stands yet 

 another sign post to mark the way. Burrowing in the sand 

 of shallow seas throughout temperate and tropical regions 

 is a little fish-like animal about two inches in length, which 

 on account of its shape has been named Amphioxus, or pointed 

 at both ends. Its world wide distribution in the face of its 

 inadequate means of dispersal, and its comparatively slight 

 specific differentiation, suggest that it is both a very ancient 

 and very conservative sort of creature. 



Running from tip to tip of the body extends a stiff rod 

 which serves as a skeleton and aids it in its rapid burrowing 

 in the sand. The body is marked by numerous V-shaped lines, 

 indicating the divisions between the segments into which the 

 muscles are divided. Just dorsal to the notochord is the hollow 

 nerve cord from which paired nerves run to various parts of 

 the body. Along this nerve cord are distributed numerous 

 little spots of pigment, which probably give the animal its 



