78 VERTEBRATE ZOOLOGY 



nerve chain and serving as a support. This and the notochord lie 

 in precisely similar positions in relation to the other organs, and in 

 both cases they are inclosed with the nerve cord in a common sheath 

 of connective tissue." 



Coming back to the idea that a vertebrate is an annelid turned over, 

 let us follow up with the aid of the diagram (Fig. 41) the results of such 

 a reversal. The annelid mouth is ventral and the oesophagus passes 

 through the brain in such a way that one pair of ganglia (supracesopha- 

 geal ganglia) are dorsal to the oesophagus while the rest of the gan- 

 glionic chain is ventral to the alimentary tract. The change from 

 annelid to vertebrate involves doing away with the anneUd or prim- 

 itive mouth and the opening up of a secondary mouth on the new 

 ventral side, which does not pass through the nervous system. The 

 entire nervous system, including the supraoesophageal ganglia (the 

 vertebrate brain) is thus left on the new dorsal side. Two pieces of 

 embryological evidence are offered in support of this contention. 

 First, the vertebrate mouth is quite late in breaking through and this 

 has been taken as a sign that it is an afterthought. Second, there are 

 vestiges of the primitive or ancestral mouth in the neuropore of ver- 

 tebrate embryos and in the adult Amphioxus, as well as in the hypo- 

 physis, a structure located where the old mouth might have been, and 

 apparently without any other significance unless it does stand for 

 the point of closure of the primitive mouth. In the annelid the blood 

 flows forward in the dorsal vessel and passes across through segmental 

 arches to a ventral vessel in which it flows backward. Reverse this 

 condition and we have the primitive vertebrate condition with the 

 blood flowing forward on the ventral side, crossing in certain special 

 arches (branchial arches) to the dorsal side and from there flowing 

 backward. Some of the annelids even have specialized branchial tis- 

 sues developed segmentally near the anterior end. In the annelid the 

 anus opens at the posterior extremity, but in the vertebrate a new 

 anus is formed on the ventral side, leaving in the embryo a blind hind- 

 gut, which subsequently disappears, leaving a part of the trunk back 

 of the anus which has no alimentary tract. This is the vertebrate 

 tail. Both the new mouth (stomodaeum) and the new anus (procto- 

 dseum) are lined with ectoderm. 



"Convincing as these comparisons seem," says Wilder, "when 

 taken by themselves, the influence of later investigation has tended 

 rather away from the annelid hypothesis, and at present, although 



