322 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



the collar region in the Enteropneusta and restricted to the truncal region in corre- 

 lation with the regional differentiation of the body, and, connected therewith, the 

 limitation of the gill-clefts (above, p. 298) 1 . 



5. The ventral coecum with chordoid walls described by Roule (Comptes Rendus, 

 t. cxxvii., 1898, p. 633) in the Actinotrocha of Phoronis sabatieri is related to the 

 ventral coecum of the stomochord of Enteropneusta. 



6. The functional oesophagus of Actinotrocha is represented by the anterior 

 portion of the body of the stomochord in Enteropneusta. 



7. Thus Actinotrocha and Cephalodiseus appear to retain in a functional con- 

 dition a portion of the gut which in the Enteropneusta has become, as such, vestigial. 

 That sessile forms should retain some primitive features in comparison with their free- 

 living relatives is not without precedent, 



The Pterobranchia are to the Enteropneusta what the Ascidiaus are to Amphioxus. 



Apart from its ventral position, the pygochord seems to me to represent what 

 must have been the condition of the notochord at its first inception ; namely, the 

 notochord was at first a longitudinal dorsal band-like thickening of the gut-wall with 

 dilated distal border; and the subnotochordal rod represents the longitudinal peduncle of 

 the longitudinal notochord. This explanation of the subnotochordal rod was suggested 

 to me by the behaviour of the pygochord with its constrictions (cf. PI. XXIX. Fig. 15 

 and PI. XXX. Fig. 35), and it is, I think, the third explanation which has been 

 suggested in recent years. 



Stohr 2 thought that the hypochorda resulted from the fusion of segmental diver- 

 ticula of the dorsal wall of the intestine. Klaatsch 3 thinks that the hypochorda is 

 the vestige of the hyperpharyngeal groove of Amphioxus'. 



VIII. Branchial Bars. 



One of the organic changes which accompanied the (hypothetical) change of function 

 of the gill-clefts (i.e. from their primary function of promoting intergonadial currents 

 of water to aerate the gonads to their secondary function of promoting the respiration 

 of the individual) was the development of tongue-bars as the essential organs of 

 respiration. It has already been pointed out that the tongue-bars of Enteropneusta 

 are not (ontogenetically) secondary as they are in Amphioxus. 



1 The gill-clefts have been limited both anteriorly and posteriorly. The anterior limitation, behind the collar- 

 region, is constant in all Enteropneusta; the posterior limitation is, as we have seen, excessively variable. 

 In connection with the hypothesis that the lateral pouches of the stomochord are the vestiges of a pair of 

 collar gill-clefts, it is useful to remember that in the larva of Amphioxus, the first gill-cleft does actually close 

 up and disappear. 



- Ph. Stohr, " TJeber die Entwicklung der Hypochorda und des dorsalen Pankreas bei Rana temporaria," 

 Morph. Jnhr. xxin. 1895, p. 123. 



3 H. Klaatsch, " Zur Frage nach der morphologischen Bedeutung der Hypochorda," Ibid. xxv. 1898, p. 156. 



4 According to my view the absence of a subnotochordal rod in Amphioxus is due to abbreviation of 

 development, i.e. it is a cenogenetic loss like the absence of medullary folds, etc. It may be remembered that 

 Eisig compared the subnotochordal rod with the " Nebendarm " of Capitellidae. 



