XVII 



PEOTOCHOEDATA 



579 



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

 median dorsal, forwardly-directed 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 lie two chitinous rods secreted by oesophageal epithelium. 

 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 applied 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.'b, 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 alive 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 



FIG. 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. 



