THE ANCESTRY OF THE VERTEBRATES 527 



needs of this particular animal, and is thus not ancestral in 

 character. 



Where the two lateral folds that form the ' peribranchial 

 chamber come together they unite in such a way as to leave 

 their original edges in the form of a pair of parallel metapleural 

 folds, and it is quite possible that either these, or more probably 

 the original folds before they extended far enough to unite 

 ventrally, are identical with those folds from which, at some 

 period in the long history between Amphioxus and the fishes, 

 the two pairs of limbs were originally derived. [Cf. Chap- 

 ter V.] 



Of the internal organs the most conspicuous is the noto- 

 chord. This is an elastic skeletal rod of gelatinous tissue, sur- 

 rounded by a firm connective tissue sheath. It extends to the 

 extreme ends of the animal and insures to it a certain grade of 

 rigidity while allowing the body to be extremely flexible. 

 Lying along the dorsal aspect of this rod, and enclosed in a 

 sheath which is continuous with that of the notochord, is the 

 dorsal nervous system, closely resembling that of fishes, but 

 with no brain other than a slight club-shaped enlargement at 

 the anterior end, the archencephalon. An olfactory pit on the 

 left side, and a median pigment spot, are its only definite sense- 

 organs. Beneath the notochord lies the alimentary canal, 

 which expands beyond the mouth into a pharynx, that extends 

 more than half the length of the body and passes into a straight 

 intestine with no especial differentiation of parts. The pharynx 

 is perforated by 60-80 pairs of narrow gill-slits, placed ob- 

 liquely, and kept open by an elaborate system of skeletal rods, 

 formed of a material resembling chitin, and thus more like 

 an invertebrate than a vertebrate structure. Both the elab- 

 orateness of this skeletal system and the very large number of 

 gill-slits are plainly secondary modifications, like the peri- 

 branchial chamber, since they are not found in the larva, and 

 mark Amphioxus as a much modified form, probably that one 

 out of a large class which survived on account of these very 

 modifications. 



A characteristic structure runs along the floor of the 



