WITH NOTES ON THE WEST INDIAN SPECIES. 301 



D. Oxygenation of the Gonads. 



By pressing the gill-slits at their first origin into direct functional relation with 

 the gonads we only assign to them their pre-eminent importance in the economy of 

 the higher animals and to that extent we explain their persistency. There is no 

 need to insist upon the truism that the possibility of progressive evolution is dependent 

 upon the gonads being exposed to the best possible physiological conditions. 



In my previous paper on Spengelia (loc. cit.) I stated that the dermal pits apparently 

 served for the irrigation of the gonads. At that time I had no idea of framing an 

 elaborate theory of gill-slits. 



What induced me to venture upon such treacherous ground was the zonulation 

 of the gonads in the genital pleurae of Pt. flava and the reflection brought home 

 by the exposed pharynx of this species that the septal bars corresponded, in principle, 

 with the annular ridges of the body-wall while the gill-clefts took the place of the 

 interannular depressions 1 (PI. XXVI. Figs. 1, 2). 



E. Emancipation of the Gonads. Lateral Septa. 



The primary relation of gill-slits and gonads which the present theory assumes is 

 not exactly retained in any existing form that we know, although it is suggested 

 by many tangible facts. The gonads have become emancipated from then - direct 

 dependence upon the gill-slits for their oxygenation concomitantly with the elaboration 

 of the vascular system. This emancipation has taken place in two ways, firstly by 

 their relegation to pleural folds of the body-wall and secondly by their recession 

 from the branchial region. 



The regional differentiation of the body is one of the cardinal features in the 

 organisation of the Enteropneusta, but it is not complete since the regions tend to 

 overlap. But whereas it is usually possible to define a genital region between branchial 

 and hepatic regions, in the subgenus Chlarnydothorax this is not possible; because, 

 although there is a short tract intervening between the pharynx and the hepatic 

 region, the gonads have no more to do with this tract than they have with any 

 other portion of the body proper. In this subgenus the gonads have become abstracted 

 from the main trunk and are confined to special pleural folds. That the genital pleurae 

 are structures superadded to the main body of the animal is evident from the behaviour 

 of the lateral septa which are an exclusive appanage of the Ptychoderidae. 



Where the genital pleurae are at their maximum, namely in Chlarnydothorax, the 

 lateral septa likewise have their greatest development. In Pt. flava the lateral septa 

 are coextensive with the genital pleurae because the latter are quite independent of 

 the branchial region. In other forms the lateral septa cease in the posterior portion 

 of the branchial region. The lateral septa are vascular folds of the basement membrane 



1 This impression is made all the stronger by the fact, already mentioned, that the tongue-bars are not 

 visible in the external view of the pharynx of preserved specimens owing to their deeper position. Only the 

 septal bars and the intervening slits are visible in such a view. 



