XVII 



PEOTOCHOEDATA 



579 



About this time, from the anterior end of the oesophagus a 



median dorsal, forwardly-direoted pouch grows out; this is the 



rudiment of the notochord (Fig. 424). Transverse sections show 



that this pouch is continued backwards as a dorsal section of the 



oesophagus, separated by lateral grooves from the rest, and that in 



these grooves He two chitinous rods secreted by oesophageal epithehum 



These are the legs of the collar 



skeleton, which is merely a 



specially thickened portion of 



the cuticle secreted by the bases 



of the notochordal cells, and 



corresponds to the primary 



cuticular sheath of the notochord 



of higher Vertebrata. The gill 



pouches become apphed to the 



ectoderm, and here the external 



gill openings are formed. Tongue 



bars, i.e. vertical folds of the dor- 

 sal walls of these pouches, dividing 



these cavities almost into two, 



are formed before the external 



openings appear (t.h, Fig. 422, C). 

 The larva has now almost 



assumed the form of the adult, 



but a remarkable change in the 



dimensions of the gut has yet 



to make its appearance. This 



has been described as a " pulling 



in" of the anterior portion of 



the gut ; but it might be more 

 aptly described as a lengthening 



of the region in front of the 

 gills. As a consequence the gills 



become pushed back till they 

 lie behind the collar region, the 

 diaphragm separating stomach 

 and intestine disappears, and 

 these two regions are conse- 

 quently no longer distinguishable. 

 Morgan was able to keep his 

 oldest Tornaria larvae ahve for 



three days after they had been caught, and to watch them metamor- 

 phose as has been described, but he did not see them begin to burrow. 

 Subsequently he visited the Bahamas and found there, in the Plankton, 

 two 'much larger varieties of Tornaria, the metamorphosis of one of 

 which (the Nassau Tornaria) he was able to study in some detail. 

 His results are recorded in a second paper (1894). 



This larva differs from the New England one, not only in its much 



Fio. 423. — Illustrating the development of 

 the dorsal nervous system in the meta- 

 morphosing Bahamas Tornaria. (After 

 Morgan. ) 



A, cross-section through the nervous system of 

 a younger specimen — the nerve plate is flanked by 

 two ectodermic folds. B, cross -section through 

 the nervous system of an older specimen — the 

 ectodermic folds have met above the nerve plate 

 and the nerve plate has become a nerve tube, n.p, 

 nerve plate ; n.t, nerve tube. 



